| 90 | ÷ | 6 | − | x | = | x | − | 90 | ÷ | 6 | + | 4 |
| Left side of the equation = | 15 | − | x |
| 15 | − | x | = | x | − | 90 | ÷ | 6 | + | 4 |
| Right side of the equation = | x | − | 15 | + | 4 |
| = | x | − | 11 |
| 15 | − | x | = | x | − | 11 |
| - | x | − | x | = | - | 11 | − | 15 |
| - | 2 | x | = | - | 11 | − | 15 |
| - | 2 | x | = | - | 26 |
| 26 | = | 2 | x |
| 2 | x | = | 26 |
| x | = | 26 | ÷ | 2 |
| = | 26 | × | 1 2 |
| = | 13 | × | 1 |
| x | = | 13 |