| ( | 15 | − | x | ) | ( | 200 | + | 50 | x | ) | = | 3500 |
| Left side of the equation = | 15 | ( | 200 | + | 50 | x | ) | − | x | ( | 200 | + | 50 | x | ) |
| = | 15 | × | 200 | + | 15 | × | 50 | x | − | x | ( | 200 | + | 50 | x | ) |
| = | 3000 | + | 750 | x | − | x | ( | 200 | + | 50 | x | ) |
| = | 3000 | + | 750 | x | − | x | × | 200 | − | x | × | 50 | x |
| = | 3000 | + | 550 | x | − | x | × | 50 | x |
| 3000 | + | 550 | x | − | x | × | 50 | x | = | 3500 |