What is the answer to the mathematical equation m=t-r for r?

Just do the work, you won't learn anything otherwise. It's also not difficult for a teacher to tell you're copying answers.

There are various ways that a mathematical expression fails to be an equation. It lacks a relational operator (symbol). It has a less than sign, or a greater than sign, or a combination of other symbols. If you language homework requires(?) that you find an antonym for equation, try non-equation.

I believe some minimal amount of knowledge of the concept will go a long way to teach you any subject.
When you say solve in Math, you mean solve an equation. An equation is a mathematical expression (with some unknown) that is equal to some other expression, usually a known value. That requires an = sign. However you do not have one.
All you can do is write the expression in as compact a form as possible. In this case, you combine the two like-terms that have the variable n to get 3n.
You expression is therefore 3n-3000. Period.

n mathematics, an expression is a finite combination of symbols that are well-formed according to the rules applicable in the context at hand. Symbols can designate values (constants), variables, operations, relations, or can constitute punctuation or other syntactic entities.
Any "expression" that represents a numeric value. Example: 2+2=4 or x+7=10. Actually the examples above are equations, not expressions. Expressions do not have = signs. 7a and 4x are examples.
it is a mathematical statement e.g 7=6+1 THE WHOLE THING IS AN EQUATION but the 6+1 is a expression.You can only have expression is there is an = sign
Yes the 6 + 1 part is an expression. Once you have an = sign or any of the other inequality symbols (,...) it is no longer an expression but a full math sentence

Algebra is the branch of mathematics concerning the study of the rules of operations and relations, and the constructions and concepts arising from them, including terms, polynomials, equations and algebraic structures. Together with geometry, analysis, topology, combinatorics, and number theory, algebra is one of the main branches of pure mathematics.
Factors and expansions a, b real; n positive integer (a ± b)² = a² ± 2ab + b²
(a ± b)³ = a³ ± 3a²b + 3ab² ± b³
(a ± b)4 = a4 ± 4a³b + 6a²b² ± 4ab³ + b4
a² - b² = (a - b)(a + b)
a² + b² = (a - bi)(a + bi)
a³ - b³ = (a - b)(a² + ab + b²)
a³ + b³ = (a + b)(a² - ab + b²)
a4 + b4 = (a² + 2½ab + b²)(a² - 2½ab + b²)
an - bn = (a - b)(an-1 + an-2b + ... + bn-1), for n even
an + bn = (a + b)(an-1 - an-2b - ... - bn-1), for n odd

Definition

A mathematical statement used to evaluate a value. An equation can use any combination of mathematical operations, including addition, subtraction, division, or multiplication. An equation can be already established due to the properties of numbers (2 + 2 = 4), or can be filled solely with variables which can be replaced with numerical values to get a resulting value. For example, the equation to calculate return on sales is: Net income ÷ Sales revenue = Return on Sales. When the values for net income and sales revenue are plugged into the equation, you are able to calculate the value of return on sales.

There are many types of mathematical equations.

1. Linear Equations y= mx + b (standard form of linear equation)
2. Quadratic Equations y= ax^2+bx+c
3. Exponential Equations y= ab^x
4. Cubic Equations y=ax^3+ bx^2+cx+d
5. Quartic Equations y= ax^4+ bx^3+ cx^2+ dx+ e
6. Equation of a circle (x-h)^2+(y-k)^2= r^2
7. Constant equation y= 9 (basically y has to equal a number for it to be a constant equation).
8. Proportional equations y=kx; y= k/x, etc.

what problem?

The equation of the line that passes through the points
(5,3) and (-7,7) is

y= (-1/3x) + (14/3)

In mathematics, a Fourier series decomposes any periodic function or periodic signal into the sum of a (possibly infinite) set of simple oscillating functions, namely sines and cosines (or complex exponentials). The study of Fourier series is a branch of Fourier analysis. Fourier series were introduced by Joseph Fourier (1768–1830) for the purpose of solving the heat equation in a metal plate.

http://en.wikipedia.org/wiki/Fourier_series

If in doubt... always GOOGLE

What you need is a software that can perform what is called "symbolic calculus". For example to give you the answer on how looks the solution for the 2 grade equation a*x^2+b*x+c=0. There are two main software packages that can perform this: Maple (http://www.maplesoft.com) and Mathematica (http://www.wolfram.com).
For simple math questions you may also want to check: www.wolframalpha.com