Steps to solve:
-15 + 12x = 5x + 7 - 3x
~Combine like terms
-15 + 12x = (5x - 3x) + 7
~Simplify
-15 + 12x = 2x + 7
~Add 15 to both sides
-15 + 15 + 12x = 2x + 7 + 15
~Simplify
12x = 2x + 22
~Subtract 2x to both sides
12x - 2x = 2x - 2x + 22
~Simplify
10x = 22
~Divide 10 to both sides
10x/10 = 22/10
~Simplify
x = 11/5 or 2.2
Best of Luck!