| ( | x | − | 40 | ) | ( | - | 20 | x | + | 1400 | ) | = | 4320 |
| Left side of the equation = | x | ( | - | 20 | x | + | 1400 | ) | − | 40 | ( | - | 20 | x | + | 1400 | ) |
| = | - | x | × | 20 | x | + | x | × | 1400 | − | 40 | ( | - | 20 | x | + | 1400 | ) |
| = | - | x | × | 20 | x | + | 1400 | x | + | 40 | × | 20 | x | − | 40 | × | 1400 |
| = | - | x | × | 20 | x | + | 1400 | x | + | 800 | x | − | 56000 |
| = | - | x | × | 20 | x | + | 2200 | x | − | 56000 |
| - | x | × | 20 | x | + | 2200 | x | − | 56000 | = | 4320 |