| ( | 12 | x | − | 8 | − | 10 | x | ) | − | ( | 8 | x | + | 11 | + | 7 | x | ) | = | 20 |
| 方程左边 = | 12 | x | − | 8 | − | 10 | x | − | ( | 8 | x | + | 11 | + | 7 | x | ) |
| = | 2 | x | − | 8 | − | ( | 8 | x | + | 11 | + | 7 | x | ) |
| = | 2 | x | − | 8 | − | 8 | x | − | 11 | − | 7 | x |
| = | - | 13 | x | − | 19 |
| - | 13 | x | − | 19 | = | 20 |
| - | 13 | x | = | 20 | + | 19 |
| - | 13 | x | = | 39 |
| - | 39 | = | 13 | x |
| 13 | x | = | - | 39 |
| x | = | - | 39 | ÷ | 13 |
| = | - | 39 | × | 1 13 |
| = | - | 3 | × | 1 |
| x | = | - | 3 |