| ( | - | 10 | x | + | 600 | ) | ( | x | − | 20 | ) | = | 3840 |
| Left side of the equation = | - | 10 | x | ( | x | − | 20 | ) | + | 600 | ( | x | − | 20 | ) |
| = | - | 10 | x | x | + | 10 | x | × | 20 | + | 600 | ( | x | − | 20 | ) |
| = | - | 10 | x | x | + | 200 | x | + | 600 | ( | x | − | 20 | ) |
| = | - | 10 | x | x | + | 200 | x | + | 600 | x | − | 600 | × | 20 |
| = | - | 10 | x | x | + | 200 | x | + | 600 | x | − | 12000 |
| = | - | 10 | x | x | + | 800 | x | − | 12000 |
| - | 10 | x | x | + | 800 | x | − | 12000 | = | 3840 |