| ( | x | − | 20 | ) | ( | - | 10 | x | + | 340 | ) | = | 400 |
| Left side of the equation = | x | ( | - | 10 | x | + | 340 | ) | − | 20 | ( | - | 10 | x | + | 340 | ) |
| = | - | x | × | 10 | x | + | x | × | 340 | − | 20 | ( | - | 10 | x | + | 340 | ) |
| = | - | x | × | 10 | x | + | 340 | x | + | 20 | × | 10 | x | − | 20 | × | 340 |
| = | - | x | × | 10 | x | + | 340 | x | + | 200 | x | − | 6800 |
| = | - | x | × | 10 | x | + | 540 | x | − | 6800 |
| - | x | × | 10 | x | + | 540 | x | − | 6800 | = | 400 |