| ( | 12 | − | 2 | y | ) | ( | 15 | − | y | ) | = | 104 |
| Left side of the equation = | 12 | ( | 15 | − | y | ) | − | 2 | y | ( | 15 | − | y | ) |
| = | 12 | × | 15 | − | 12 | y | − | 2 | y | ( | 15 | − | y | ) |
| = | 180 | − | 12 | y | − | 2 | y | ( | 15 | − | y | ) |
| = | 180 | − | 12 | y | − | 2 | y | × | 15 | + | 2 | y | y |
| = | 180 | − | 12 | y | − | 30 | y | + | 2 | y | y |
| = | 180 | − | 42 | y | + | 2 | y | y |
| 180 | − | 42 | y | + | 2 | y | y | = | 104 |