If you would like to calculate 1/2 * a + (-3 * a) - 3 * b - 0.7 * a, you can do this using the following steps:
1/2 * a + (-3 * a) - 3 * b - 0.7 * a = 0.5 * a - 3 * a - 0.7 * a - 3 * b = - 3.2 * a - 3 * b
The correct result would be - 3.2 * a - 3 * b.