count the number of x's. there are three positive x's (3x = 3*x = x+x+x), and 3 negatives (-12x = -12*x = -x-x-x-x-x-x....., well, you get it). Together, they are x+x+x -x-x-x-x-x-x-x-x-x-x-x-x, or 9x, because the positive and negative x's cancel. (the space signifies nothing).
