| ( | 30 | − | x | ) | ( | 60 | + | 20 | x | ) | = | 5280 |
| Left side of the equation = | 30 | ( | 60 | + | 20 | x | ) | − | x | ( | 60 | + | 20 | x | ) |
| = | 30 | × | 60 | + | 30 | × | 20 | x | − | x | ( | 60 | + | 20 | x | ) |
| = | 1800 | + | 600 | x | − | x | ( | 60 | + | 20 | x | ) |
| = | 1800 | + | 600 | x | − | x | × | 60 | − | x | × | 20 | x |
| = | 1800 | + | 540 | x | − | x | × | 20 | x |
| 1800 | + | 540 | x | − | x | × | 20 | x | = | 5280 |