Distribute the 4 throughout the set of parenthesis on the right
4 · 2x = 8x
4 · -3 = -12
The equation now looks like:
8x - 12 = 8x - 12
Subtract the right hand 8x on both sides of the equation
8x - 8x = 0x = 0
-12 = -12
This means that there is infinite solutions