| ( | 2 | − | x | ) | ( | 400 | − | 20 | x | ) | = | 2240 |
| Left side of the equation = | 2 | ( | 400 | − | 20 | x | ) | − | x | ( | 400 | − | 20 | x | ) |
| = | 2 | × | 400 | − | 2 | × | 20 | x | − | x | ( | 400 | − | 20 | x | ) |
| = | 800 | − | 40 | x | − | x | ( | 400 | − | 20 | x | ) |
| = | 800 | − | 40 | x | − | x | × | 400 | + | x | × | 20 | x |
| = | 800 | − | 440 | x | + | x | × | 20 | x |
| 800 | − | 440 | x | + | x | × | 20 | x | = | 2240 |