| ( | x | − | 10 | ) | ( | - | 30 | x | + | 900 | ) | = | 0 |
| Left side of the equation = | x | ( | - | 30 | x | + | 900 | ) | − | 10 | ( | - | 30 | x | + | 900 | ) |
| = | - | x | × | 30 | x | + | x | × | 900 | − | 10 | ( | - | 30 | x | + | 900 | ) |
| = | - | x | × | 30 | x | + | 900 | x | + | 10 | × | 30 | x | − | 10 | × | 900 |
| = | - | x | × | 30 | x | + | 900 | x | + | 300 | x | − | 9000 |
| = | - | x | × | 30 | x | + | 1200 | x | − | 9000 |
| - | x | × | 30 | x | + | 1200 | x | − | 9000 | = | 0 |