26,629,900,605 mJ = ?
I prefer to write this large quantity in standard exponential notation: 2.6629900605 x 10^10 mJ.
A method called dimensional analysis is ideal for a problem like this.
Use the following equivalencies:
1 J = 10^3 mJ (also written as 1 J = E3 mJ, OR 1 J = 1000 mJ).
1 MJ = 10^6 J (or 1 MJ = E6, OR 1 MJ = 1,000,000 J.)
You may rewrite them as more useful ratios, as follows:
1 J / 10^3 mJ OR 10^3 mJ / 1 J
1 MJ / 10^6 J OR 10^6 J / 1 MJ
Now, select the ratio from each set of ratios as conversion factors that allow you to cancel out like units.
Set up the conversions as parts of a convenient chain equation, as follows:
Notice that all like units cancel out, and I am left with only MJ, the desired unit. First I converted mJ to J, then J to MJ. Using exponentials makes the problem a bit more manageable.
Oh, and here is a very useful website that provides some very high quality instruction on use of dimensional analysis