Here is the problem I am working on and what I did so far:

4+m-3m
First I did this:
4+(m-3m)
Then I did this:
4+(-2m)
I thought my answer should be:
4-2m
The answer key says the answer should be:
-2m+4
What did I do wrong?

you did nothing wrong, your answer is correct,

yours is just in a different order

(isn't
3+4 = 4+3 or
+5 - 9 = -9 + 5 ?)