Cindy is 5 years older than Alex and 7 years older than peter sum

Three unknowns with three equasions:
cindy is 5 years older than alex. write it as c = a + 5
cindy is 7 years older than peter. write it as c = p + 7
The sum of their ages is 21. write it as c + a + p = 21
first solve for peters age in terms of alex's age.
c = p + 7 or
(a + 5) = p + 7 or
p = (a + 5) - 7 or
p = a - 2
substitute in and solve the equasion:
(a + 5) + a + (a - 2) = 21 or
3a + 3 = 21 or
3a = 24 or
a = 8

c = a + 5 or
c = 8 + 5 = 13

c = p + 7 or
p = c - 7 or
p = 13 - 7 = 6

cindy is 13, alex is 8 and peter is 6
This is called simultaneous equasions and will always work if you have the same number of unknowns as you have equasions (or relationships).