Method One
You could divide by 5 right away.
20/5 = - 3 + x
4 = - 3 + x Add 3 to both sides.
4 + 3 = x
x = 7
Method Two.
You could remove the brackets. This is slightly longer, but some questions can only be solved that way.
20 = 5(-3) + 5x
20 = -15 + 5x Add 15 to both sides
20 + 15 = 5x
35 = 5x Divide by 5
35/5 = x
7 = x