| 3 | ( | - | x | − | 13 | ) | + | 7 | ( | - | 7 | x | − | 12 | ) | + | 1 | = | - | 18 |
| 方程左边 = | - | 3 | x | − | 3 | × | 13 | + | 7 | ( | - | 7 | x | − | 12 | ) | + | 1 |
| = | - | 3 | x | − | 39 | + | 7 | ( | - | 7 | x | − | 12 | ) | + | 1 |
| = | - | 3 | x | − | 38 | + | 7 | ( | - | 7 | x | − | 12 | ) |
| = | - | 3 | x | − | 38 | − | 7 | × | 7 | x | − | 7 | × | 12 |
| = | - | 3 | x | − | 38 | − | 49 | x | − | 84 |
| = | - | 52 | x | − | 122 |
| - | 52 | x | − | 122 | = | - | 18 |
| - | 52 | x | = | - | 18 | + | 122 |
| - | 52 | x | = | 104 |
| - | 104 | = | 52 | x |
| 52 | x | = | - | 104 |
| x | = | - | 104 | ÷ | 52 |
| = | - | 104 | × | 1 52 |
| = | - | 2 | × | 1 |
| x | = | - | 2 |