If you would like to calculate A + B and B - A, you can do this using the following steps:
A = 3 * X - 2
B = - 6 * X - 7
A + B = (3 * X - 2) + (- 6 * X - 7) = - 3 * X - 9
B - A = (- 6 * X - 7) - (3 * X - 2) = - 9 * X - 5
The correct result would be B.