| ( | x | − | 40 | ) | ( | 2100 | − | 30 | x | ) | = | 6480 |
| 方程左边 = | x | ( | 2100 | − | 30 | x | ) | − | 40 | ( | 2100 | − | 30 | x | ) |
| = | x | × | 2100 | − | x | × | 30 | x | − | 40 | ( | 2100 | − | 30 | x | ) |
| = | 2100 | x | − | x | × | 30 | x | − | 40 | × | 2100 | + | 40 | × | 30 | x |
| = | 2100 | x | − | x | × | 30 | x | − | 84000 | + | 1200 | x |
| = | 3300 | x | − | x | × | 30 | x | − | 84000 |
| 3300 | x | − | x | × | 30 | x | − | 84000 | = | 6480 |