| ( | 18 | − | x | ) | ( | 500 | + | 50 | x | ) | = | 0 |
| Left side of the equation = | 18 | ( | 500 | + | 50 | x | ) | − | x | ( | 500 | + | 50 | x | ) |
| = | 18 | × | 500 | + | 18 | × | 50 | x | − | x | ( | 500 | + | 50 | x | ) |
| = | 9000 | + | 900 | x | − | x | ( | 500 | + | 50 | x | ) |
| = | 9000 | + | 900 | x | − | x | × | 500 | − | x | × | 50 | x |
| = | 9000 | + | 400 | x | − | x | × | 50 | x |
| 9000 | + | 400 | x | − | x | × | 50 | x | = | 0 |