| ( | y | − | 60 | ) | ( | 220 | − | 2 | y | ) | = | 1200 |
| Left side of the equation = | y | ( | 220 | − | 2 | y | ) | − | 60 | ( | 220 | − | 2 | y | ) |
| = | y | × | 220 | − | y | × | 2 | y | − | 60 | ( | 220 | − | 2 | y | ) |
| = | 220 | y | − | y | × | 2 | y | − | 60 | × | 220 | + | 60 | × | 2 | y |
| = | 220 | y | − | y | × | 2 | y | − | 13200 | + | 120 | y |
| = | 340 | y | − | y | × | 2 | y | − | 13200 |
| 340 | y | − | y | × | 2 | y | − | 13200 | = | 1200 |