If you would like to calculate 0.4 * x - 0.8 - 3 * x + 0.5 * x - 1.2, you can do this using the following steps:
0.4 * x - 0.8 - 3 * x + 0.5 * x - 1.2 = (0.4 * x - 3 * x + 0.5 * x) - 0.8 - 1.2 = - 2.1 * x - 2
The correct result would be - 2.1 * x - 2.