| - | ( | 20 | − | x | ) | ( | 40 | + | x | ) | − | 900 | = | 0 |
| Left side of the equation = | - | 20 | ( | 40 | + | x | ) | + | x | ( | 40 | + | x | ) | − | 900 |
| = | - | 20 | × | 40 | − | 20 | x | + | x | ( | 40 | + | x | ) | − | 900 |
| = | - | 800 | − | 20 | x | + | x | ( | 40 | + | x | ) | − | 900 |
| = | - | 1700 | − | 20 | x | + | x | ( | 40 | + | x | ) |
| = | - | 1700 | − | 20 | x | + | x | × | 40 | + | x | x |
| = | - | 1700 | + | 20 | x | + | x | x |
| - | 1700 | + | 20 | x | + | x | x | = | 0 |