Let's simplify the equation step by step:

X - 15 + 8x - (13x + 16.5) = 12x

X - 15 + 8x - 13x - 16.5 = 12x
X - 15 + 8x - 13x - 16.5 = 12x
-5x - 31.5 = 12x

Now, let's isolate the variable x by moving all terms without x to the other side of the equation:

-5x - 31.5 = 12x
-5x - 12x = 31.5
-17x = 31.5

Now, divide both sides by -17 to solve for x:

x = -31.5 / 17
x = -1.85

Therefore, the solution to the equation X - 15 + 8x - (13x + 16.5) = 12x is x = -1.85.