| - | 3 | ( | - | 9 | a | − | 9 | ) | − | 4 | ( | 8 | a | − | 10 | ) | = | 0 |
| 方程左边 = | 3 | × | 9 | a | + | 3 | × | 9 | − | 4 | ( | 8 | a | − | 10 | ) |
| = | 27 | a | + | 27 | − | 4 | ( | 8 | a | − | 10 | ) |
| = | 27 | a | + | 27 | − | 4 | × | 8 | a | + | 4 | × | 10 |
| = | 27 | a | + | 27 | − | 32 | a | + | 40 |
| = | - | 5 | a | + | 67 |
| - | 5 | a | + | 67 | = | 0 |
| - | 5 | a | = | 0 | − | 67 |
| - | 5 | a | = | - | 67 |
| 67 | = | 5 | a |
| 5 | a | = | 67 |
| a | = | 67 | ÷ | 5 |
| = | 67 | × | 1 5 |
| a | = | 67 5 |
| a | = | 13.4 |