| ( | x | − | 40 | ) | ( | 2 | x | − | 100 | ) | = | 400 |
| Left side of the equation = | x | ( | 2 | x | − | 100 | ) | − | 40 | ( | 2 | x | − | 100 | ) |
| = | x | × | 2 | x | − | x | × | 100 | − | 40 | ( | 2 | x | − | 100 | ) |
| = | x | × | 2 | x | − | 100 | x | − | 40 | × | 2 | x | + | 40 | × | 100 |
| = | x | × | 2 | x | − | 100 | x | − | 80 | x | + | 4000 |
| = | x | × | 2 | x | − | 180 | x | + | 4000 |
| x | × | 2 | x | − | 180 | x | + | 4000 | = | 400 |