| ( | x | − | 10 | ) | ( | 220 | − | 10 | x | ) | = | 1800 |
| Left side of the equation = | x | ( | 220 | − | 10 | x | ) | − | 10 | ( | 220 | − | 10 | x | ) |
| = | x | × | 220 | − | x | × | 10 | x | − | 10 | ( | 220 | − | 10 | x | ) |
| = | 220 | x | − | x | × | 10 | x | − | 10 | × | 220 | + | 10 | × | 10 | x |
| = | 220 | x | − | x | × | 10 | x | − | 2200 | + | 100 | x |
| = | 320 | x | − | x | × | 10 | x | − | 2200 |
| 320 | x | − | x | × | 10 | x | − | 2200 | = | 1800 |