| ( | 28 | − | x | ) | ( | 20 | − | x | ) | = | 560 | × | 3 4 |
| Left side of the equation = | 28 | ( | 20 | − | x | ) | − | x | ( | 20 | − | x | ) |
| = | 28 | × | 20 | − | 28 | x | − | x | ( | 20 | − | x | ) |
| = | 560 | − | 28 | x | − | x | ( | 20 | − | x | ) |
| = | 560 | − | 28 | x | − | x | × | 20 | + | x | x |
| = | 560 | − | 48 | x | + | x | x |
| 560 | − | 48 | x | + | x | x | = | 560 | × | 3 4 |
| Right side of the equation = | 420 |
| 560 | − | 48 | x | + | x | x | = | 420 |