| ( | x | − | 40 | ) | ( | - | 2 | x | + | 140 | ) | = | 450 |
| Left side of the equation = | x | ( | - | 2 | x | + | 140 | ) | − | 40 | ( | - | 2 | x | + | 140 | ) |
| = | - | x | × | 2 | x | + | x | × | 140 | − | 40 | ( | - | 2 | x | + | 140 | ) |
| = | - | x | × | 2 | x | + | 140 | x | + | 40 | × | 2 | x | − | 40 | × | 140 |
| = | - | x | × | 2 | x | + | 140 | x | + | 80 | x | − | 5600 |
| = | - | x | × | 2 | x | + | 220 | x | − | 5600 |
| - | x | × | 2 | x | + | 220 | x | − | 5600 | = | 450 |