| ( | x | − | 30 | ) | ( | - | 10 | x | + | 700 | ) | = | 3600 |
| Left side of the equation = | x | ( | - | 10 | x | + | 700 | ) | − | 30 | ( | - | 10 | x | + | 700 | ) |
| = | - | x | × | 10 | x | + | x | × | 700 | − | 30 | ( | - | 10 | x | + | 700 | ) |
| = | - | x | × | 10 | x | + | 700 | x | + | 30 | × | 10 | x | − | 30 | × | 700 |
| = | - | x | × | 10 | x | + | 700 | x | + | 300 | x | − | 21000 |
| = | - | x | × | 10 | x | + | 1000 | x | − | 21000 |
| - | x | × | 10 | x | + | 1000 | x | − | 21000 | = | 3600 |