Find the sum or difference of (2m+-4m)

(2m + 7) - (3 - 4m) 

First distribute the (-) in front of the quantity of 3 - 4m. 
Think of it as this: 

-1(3 - 4m) 

You multiply each number inside the parenthesis by -1. 

So: 

3*-1 = -3
-4m * -1 = 4m

Now that you've distributed the negative you can remove the parenthesis. 
Now you'll just set it all up like you would any normal problem. Because -3 is negative you'll make it subtract I'll show you what I mean: 

2m + 7 - 3 +  4m 

Combine like terms (Terms which have the same variables... in this case the same number of m's)

Rearranging them can help!

4m + 2m + 7 - 3 

6m + 7 - 3

6m + 4, is our difference!