| ( | 9000 | − | 450 | x | ) | ( | 31 | + | x | ) | = | 0 |
| 方程左边 = | 9000 | ( | 31 | + | x | ) | − | 450 | x | ( | 31 | + | x | ) |
| = | 9000 | × | 31 | + | 9000 | x | − | 450 | x | ( | 31 | + | x | ) |
| = | 279000 | + | 9000 | x | − | 450 | x | ( | 31 | + | x | ) |
| = | 279000 | + | 9000 | x | − | 450 | x | × | 31 | − | 450 | x | x |
| = | 279000 | + | 9000 | x | − | 13950 | x | − | 450 | x | x |
| = | 279000 | − | 4950 | x | − | 450 | x | x |
| 279000 | − | 4950 | x | − | 450 | x | x | = | 0 |