| ( | a | − | 20 | ) | ( | - | 2 | a | + | 80 | ) | = | 150 |
| Left side of the equation = | a | ( | - | 2 | a | + | 80 | ) | − | 20 | ( | - | 2 | a | + | 80 | ) |
| = | - | a | × | 2 | a | + | a | × | 80 | − | 20 | ( | - | 2 | a | + | 80 | ) |
| = | - | a | × | 2 | a | + | 80 | a | + | 20 | × | 2 | a | − | 20 | × | 80 |
| = | - | a | × | 2 | a | + | 80 | a | + | 40 | a | − | 1600 |
| = | - | a | × | 2 | a | + | 120 | a | − | 1600 |
| - | a | × | 2 | a | + | 120 | a | − | 1600 | = | 150 |