| ( | x | − | 100 | ) | ( | - | x | + | 180 | ) | = | 1600 |
| Left side of the equation = | x | ( | - | x | + | 180 | ) | − | 100 | ( | - | x | + | 180 | ) |
| = | - | x | x | + | x | × | 180 | − | 100 | ( | - | x | + | 180 | ) |
| = | - | x | x | + | 180 | x | + | 100 | x | − | 100 | × | 180 |
| = | - | x | x | + | 180 | x | + | 100 | x | − | 18000 |
| = | - | x | x | + | 280 | x | − | 18000 |
| - | x | x | + | 280 | x | − | 18000 | = | 1600 |