first of all, you would use the FOIL method for -5(a + 3). This means to just multiply -5 to all of the values in the parenthesis. This would equal:
-5a -15 = -55
Then after, you would add 15 for both sides and get -5a = -40
Divide by -5 on both sides, you get a = 8.
Here is a visual representation: https://puu.sh/zPxLC/a16740d0ce.png
