| 3120 | ÷ | ( | x | − | 9 | ) | = | 4200 | ÷ | x |
| Multiply both sides of the equation by: | ( | x | − | 9 | ) | , | x |
| 3120 | x | = | 4200 | ( | x | − | 9 | ) |
| 3120 | x | = | 4200 | x | − | 4200 | × | 9 |
| 3120 | x | = | 4200 | x | − | 37800 |
| 3120 | x | − | 4200 | x | = | - | 37800 |
| - | 1080 | x | = | - | 37800 |
| 37800 | = | 1080 | x |
| 1080 | x | = | 37800 |
| x | = | 37800 | ÷ | 1080 |
| = | 37800 | × | 1 1080 |
| = | 35 | × | 1 |
| x | = | 35 |