| ( | 80 | − | x | ) | ( | 280 | − | 2 | x | ) | = | 1350 |
| Left side of the equation = | 80 | ( | 280 | − | 2 | x | ) | − | x | ( | 280 | − | 2 | x | ) |
| = | 80 | × | 280 | − | 80 | × | 2 | x | − | x | ( | 280 | − | 2 | x | ) |
| = | 22400 | − | 160 | x | − | x | ( | 280 | − | 2 | x | ) |
| = | 22400 | − | 160 | x | − | x | × | 280 | + | x | × | 2 | x |
| = | 22400 | − | 440 | x | + | x | × | 2 | x |
| 22400 | − | 440 | x | + | x | × | 2 | x | = | 1350 |