| ( | x | − | 30 | ) | ( | 280 | − | 10 | ( | x | − | 30 | ) | ) | = | 3120 |
| Left side of the equation = | x | ( | 280 | − | 10 | ( | x | − | 30 | ) | ) | − | 30 | ( | 280 | − | 10 | ( | x | − | 30 | ) | ) |
| = | x | × | 280 | − | x | × | 10 | ( | x | − | 30 | ) | − | 30 | ( | 280 | − | 10 | ( | x | − | 30 | ) | ) |
| = | 280 | x | − | x | × | 10 | x | + | x | × | 10 | × | 30 | − | 30 | ( | 280 | − | 10 | ( | x | − | 30 | ) | ) |
| = | 280 | x | − | x | × | 10 | x | + | x | × | 300 | − | 30 | ( | 280 | − | 10 | ( | x | − | 30 | ) | ) |
| = | 580 | x | − | x | × | 10 | x | − | 30 | ( | 280 | − | 10 | ( | x | − | 30 | ) | ) |
| = | 580 | x | − | x | × | 10 | x | − | 30 | × | 280 | + | 30 | × | 10 | ( | x | − | 30 | ) |
| = | 580 | x | − | x | × | 10 | x | − | 8400 | + | 300 | ( | x | − | 30 | ) |
| = | 580 | x | − | x | × | 10 | x | − | 8400 | + | 300 | x | − | 300 | × | 30 |
| = | 580 | x | − | x | × | 10 | x | − | 8400 | + | 300 | x | − | 9000 |
| = | 880 | x | − | x | × | 10 | x | − | 17400 |
| 880 | x | − | x | × | 10 | x | − | 17400 | = | 3120 |