| ( | 20 | − | x | ) | × | 20 | ( | 20 | − | x | ) | = | 2880 |
| Left side of the equation = | 20 | × | 20 | ( | 20 | − | x | ) | − | x | × | 20 | ( | 20 | − | x | ) |
| = | 400 | ( | 20 | − | x | ) | − | x | × | 20 | ( | 20 | − | x | ) |
| = | 400 | × | 20 | − | 400 | x | − | x | × | 20 | ( | 20 | − | x | ) |
| = | 8000 | − | 400 | x | − | x | × | 20 | ( | 20 | − | x | ) |
| = | 8000 | − | 400 | x | − | x | × | 20 | × | 20 | + | x | × | 20 | x |
| = | 8000 | − | 400 | x | − | x | × | 400 | + | x | × | 20 | x |
| = | 8000 | − | 800 | x | + | x | × | 20 | x |
| 8000 | − | 800 | x | + | x | × | 20 | x | = | 2880 |