| ( | 80 | − | x | ) | ( | 100 | + | 4 | x | ) | = | 12000 |
| Left side of the equation = | 80 | ( | 100 | + | 4 | x | ) | − | x | ( | 100 | + | 4 | x | ) |
| = | 80 | × | 100 | + | 80 | × | 4 | x | − | x | ( | 100 | + | 4 | x | ) |
| = | 8000 | + | 320 | x | − | x | ( | 100 | + | 4 | x | ) |
| = | 8000 | + | 320 | x | − | x | × | 100 | − | x | × | 4 | x |
| = | 8000 | + | 220 | x | − | x | × | 4 | x |
| 8000 | + | 220 | x | − | x | × | 4 | x | = | 12000 |