| 400 | = | ( | 120 | − | 2 | x | ) | ( | x | − | 40 | ) |
| Right side of the equation = | 120 | ( | x | − | 40 | ) | − | 2 | x | ( | x | − | 40 | ) |
| = | 120 | x | − | 120 | × | 40 | − | 2 | x | ( | x | − | 40 | ) |
| = | 120 | x | − | 4800 | − | 2 | x | ( | x | − | 40 | ) |
| = | 120 | x | − | 4800 | − | 2 | x | x | + | 2 | x | × | 40 |
| = | 120 | x | − | 4800 | − | 2 | x | x | + | 80 | x |
| = | 200 | x | − | 4800 | − | 2 | x | x |
| 400 | = | 200 | x | − | 4800 | − | 2 | x | x |