| ( | 30 | + | x | ) | ( | 1200 | − | 50 | x | ) | = | 28000 |
| Left side of the equation = | 30 | ( | 1200 | − | 50 | x | ) | + | x | ( | 1200 | − | 50 | x | ) |
| = | 30 | × | 1200 | − | 30 | × | 50 | x | + | x | ( | 1200 | − | 50 | x | ) |
| = | 36000 | − | 1500 | x | + | x | ( | 1200 | − | 50 | x | ) |
| = | 36000 | − | 1500 | x | + | x | × | 1200 | − | x | × | 50 | x |
| = | 36000 | − | 300 | x | − | x | × | 50 | x |
| 36000 | − | 300 | x | − | x | × | 50 | x | = | 28000 |