| 12 | − | 3 | ( | a | − | 3 | ) | = | 18 | − | 7 | ( | a | − | 3 | ) |
| Left side of the equation = | 12 | − | 3 | a | + | 3 | × | 3 |
| = | 12 | − | 3 | a | + | 9 |
| = | 21 | − | 3 | a |
| 21 | − | 3 | a | = | 18 | − | 7 | ( | a | − | 3 | ) |
| Right side of the equation = | 18 | − | 7 | a | + | 7 | × | 3 |
| = | 18 | − | 7 | a | + | 21 |
| = | 39 | − | 7 | a |
| 21 | − | 3 | a | = | 39 | − | 7 | a |
| - | 3 | a | + | 7 | a | = | 39 | − | 21 |
| 4 | a | = | 39 | − | 21 |
| 4 | a | = | 18 |
| a | = | 18 | ÷ | 4 |
| = | 18 | × | 1 4 |
| = | 9 | × | 1 2 |
| a | = | 9 2 |
| a | = | 4.5 |