| ( | 10 | + | x | ) | ( | 800 | − | 100 | x | ) | = | 12000 |
| Left side of the equation = | 10 | ( | 800 | − | 100 | x | ) | + | x | ( | 800 | − | 100 | x | ) |
| = | 10 | × | 800 | − | 10 | × | 100 | x | + | x | ( | 800 | − | 100 | x | ) |
| = | 8000 | − | 1000 | x | + | x | ( | 800 | − | 100 | x | ) |
| = | 8000 | − | 1000 | x | + | x | × | 800 | − | x | × | 100 | x |
| = | 8000 | − | 200 | x | − | x | × | 100 | x |
| 8000 | − | 200 | x | − | x | × | 100 | x | = | 12000 |