| 13290 | − | 39 | x | = | 21 | ( | 10 | x | + | 40 | ) |
| Right side of the equation = | 21 | × | 10 | x | + | 21 | × | 40 |
| = | 210 | x | + | 840 |
| 13290 | − | 39 | x | = | 210 | x | + | 840 |
| - | 39 | x | − | 210 | x | = | 840 | − | 13290 |
| - | 249 | x | = | 840 | − | 13290 |
| - | 249 | x | = | - | 12450 |
| 12450 | = | 249 | x |
| 249 | x | = | 12450 |
| x | = | 12450 | ÷ | 249 |
| = | 12450 | × | 1 249 |
| = | 50 | × | 1 |
| x | = | 50 |