| ( | x | − | 20 | ) | ( | 180 | − | 3 | x | ) | = | 900 |
| Left side of the equation = | x | ( | 180 | − | 3 | x | ) | − | 20 | ( | 180 | − | 3 | x | ) |
| = | x | × | 180 | − | x | × | 3 | x | − | 20 | ( | 180 | − | 3 | x | ) |
| = | 180 | x | − | x | × | 3 | x | − | 20 | × | 180 | + | 20 | × | 3 | x |
| = | 180 | x | − | x | × | 3 | x | − | 3600 | + | 60 | x |
| = | 240 | x | − | x | × | 3 | x | − | 3600 |
| 240 | x | − | x | × | 3 | x | − | 3600 | = | 900 |