| ( | 3 | − | 3 | ÷ | a | ) | ( | - | 1 | − | a | ) | = | 12 |
| 3 | ( | - | 1 | − | a | ) | − | 3 | ÷ | a | × | ( | - | 1 | − | a | ) | = | 12 |
| 方程两边同时乘以: | a |
| 3 | ( | - | 1 | − | a | ) | a | − | 3 | ( | - | 1 | − | a | ) | = | 12 | a |
| - | 3 | × | 1 | a | − | 3 | a | a | − | 3 | ( | - | 1 | − | a | ) | = | 12 | a |
| - | 3 | a | − | 3 | a | a | − | 3 | ( | - | 1 | − | a | ) | = | 12 | a |
| - | 3 | a | − | 3 | a | a | + | 3 | × | 1 | + | 3 | a | = | 12 | a |
| - | 3 | a | − | 3 | a | a | + | 3 | + | 3 | a | = | 12 | a |
| - | 0 | a | − | 3 | a | a | + | 3 | = | 12 | a |