| ( | 18 | − | 2 | x | ) | ( | 15 | − | x | ) | = | 144 |
| Left side of the equation = | 18 | ( | 15 | − | x | ) | − | 2 | x | ( | 15 | − | x | ) |
| = | 18 | × | 15 | − | 18 | x | − | 2 | x | ( | 15 | − | x | ) |
| = | 270 | − | 18 | x | − | 2 | x | ( | 15 | − | x | ) |
| = | 270 | − | 18 | x | − | 2 | x | × | 15 | + | 2 | x | x |
| = | 270 | − | 18 | x | − | 30 | x | + | 2 | x | x |
| = | 270 | − | 48 | x | + | 2 | x | x |
| 270 | − | 48 | x | + | 2 | x | x | = | 144 |