| ( | x | − | 20 | ) | ( | 320 | − | 10 | x | ) | = | 0 |
| Left side of the equation = | x | ( | 320 | − | 10 | x | ) | − | 20 | ( | 320 | − | 10 | x | ) |
| = | x | × | 320 | − | x | × | 10 | x | − | 20 | ( | 320 | − | 10 | x | ) |
| = | 320 | x | − | x | × | 10 | x | − | 20 | × | 320 | + | 20 | × | 10 | x |
| = | 320 | x | − | x | × | 10 | x | − | 6400 | + | 200 | x |
| = | 520 | x | − | x | × | 10 | x | − | 6400 |
| 520 | x | − | x | × | 10 | x | − | 6400 | = | 0 |