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