| ( | x | − | 40 | ) | ( | 300 | + | 20 | x | ) | = | 6080 |
| Left side of the equation = | x | ( | 300 | + | 20 | x | ) | − | 40 | ( | 300 | + | 20 | x | ) |
| = | x | × | 300 | + | x | × | 20 | x | − | 40 | ( | 300 | + | 20 | x | ) |
| = | 300 | x | + | x | × | 20 | x | − | 40 | × | 300 | − | 40 | × | 20 | x |
| = | 300 | x | + | x | × | 20 | x | − | 12000 | − | 800 | x |
| = | - | 500 | x | + | x | × | 20 | x | − | 12000 |
| - | 500 | x | + | x | × | 20 | x | − | 12000 | = | 6080 |