| ( | x | + | 1 | ) | ( | - | 20 | x | + | 600 | ) | = | 2600 |
| Left side of the equation = | x | ( | - | 20 | x | + | 600 | ) | + | 1 | ( | - | 20 | x | + | 600 | ) |
| = | - | x | × | 20 | x | + | x | × | 600 | + | 1 | ( | - | 20 | x | + | 600 | ) |
| = | - | x | × | 20 | x | + | 600 | x | − | 1 | × | 20 | x | + | 1 | × | 600 |
| = | - | x | × | 20 | x | + | 600 | x | − | 20 | x | + | 600 |
| = | - | x | × | 20 | x | + | 580 | x | + | 600 |
| - | x | × | 20 | x | + | 580 | x | + | 600 | = | 2600 |