You need to isolate/get x by itself in the equation
6 + 3(6x - 2) = -12 Distribute/multiply 3 into (6x - 2)
6 + (3)6x - (3)2 = -12
6 + 18x - 6 = -12 Combine like terms (6 and -6)
18x = -12 Divide 18 on both sides to get x by itself
Simplify

PROOF
Plug in -2/3 into x, then multiply 6 and -2/3
6 + 3(-4 - 2) = -12 Simplify what's in the parentheses
6 + 3(-6) = -12 Multiply 3 and -6
6 - 18 = -12
-12 = -12