| x | = | 3 | ( | 90 | − | ( | 120 | − | x | ) | ) |
| Right side of the equation = | 3 | × | 90 | − | 3 | ( | 120 | − | x | ) |
| = | 270 | − | 3 | ( | 120 | − | x | ) |
| = | 270 | − | 3 | × | 120 | + | 3 | x |
| = | 270 | − | 360 | + | 3 | x |
| = | - | 90 | + | 3 | x |
| x | = | - | 90 | + | 3 | x |
| x | − | 3 | x | = | - | 90 |
| - | 2 | x | = | - | 90 |
| 90 | = | 2 | x |
| 2 | x | = | 90 |
| x | = | 90 | ÷ | 2 |
| = | 90 | × | 1 2 |
| = | 45 | × | 1 |
| x | = | 45 |