To solve this problem, set the expression equal to zero.
0 = 12 + 2x/6 - x. Multiply everything by 6.
0 = 72 + 2x - 6x. Combine like terms
0 = 72 - 4x. Subtract 72 from both sides.
-72 = -4x. Divide both sides by -4.
18 = x.
The rational expression = 0 when x = 18.