| ( | 30 | − | x | ) | ( | 10 | x | + | 180 | ) | = | 3200 |
| Left side of the equation = | 30 | ( | 10 | x | + | 180 | ) | − | x | ( | 10 | x | + | 180 | ) |
| = | 30 | × | 10 | x | + | 30 | × | 180 | − | x | ( | 10 | x | + | 180 | ) |
| = | 300 | x | + | 5400 | − | x | ( | 10 | x | + | 180 | ) |
| = | 300 | x | + | 5400 | − | x | × | 10 | x | − | x | × | 180 |
| = | 120 | x | + | 5400 | − | x | × | 10 | x |
| 120 | x | + | 5400 | − | x | × | 10 | x | = | 3200 |