| ( | - | x | + | 1200 | ) | ( | x | − | 200 | ) | = | 21000 |
| Left side of the equation = | - | x | ( | x | − | 200 | ) | + | 1200 | ( | x | − | 200 | ) |
| = | - | x | x | + | x | × | 200 | + | 1200 | ( | x | − | 200 | ) |
| = | - | x | x | + | 200 | x | + | 1200 | x | − | 1200 | × | 200 |
| = | - | x | x | + | 200 | x | + | 1200 | x | − | 240000 |
| = | - | x | x | + | 1400 | x | − | 240000 |
| - | x | x | + | 1400 | x | − | 240000 | = | 21000 |