| ( | 12 | x | − | 8 | − | 10 | x | ) | + | ( | 8 | x | − | 11 | − | 7 | x | ) | = | 20 |
| Left side of the equation = | 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 |
| = | 3 | x | − | 19 |
| 3 | x | − | 19 | = | 20 |
| 3 | x | = | 20 | + | 19 |
| 3 | x | = | 39 |
| x | = | 39 | ÷ | 3 |
| = | 39 | × | 1 3 |
| = | 13 | × | 1 |
| x | = | 13 |