| ( | x | − | 20 | ) | ( | 400 | − | 10 | x | ) | = | 750 |
| Left side of the equation = | x | ( | 400 | − | 10 | x | ) | − | 20 | ( | 400 | − | 10 | x | ) |
| = | x | × | 400 | − | x | × | 10 | x | − | 20 | ( | 400 | − | 10 | x | ) |
| = | 400 | x | − | x | × | 10 | x | − | 20 | × | 400 | + | 20 | × | 10 | x |
| = | 400 | x | − | x | × | 10 | x | − | 8000 | + | 200 | x |
| = | 600 | x | − | x | × | 10 | x | − | 8000 |
| 600 | x | − | x | × | 10 | x | − | 8000 | = | 750 |