| 72 | ( | x | − | 21 | ) | + | 72 | ( | x | + | 21 | ) | = | 7 |
| Left side of the equation = | 72 | x | − | 72 | × | 21 | + | 72 | ( | x | + | 21 | ) |
| = | 72 | x | − | 1512 | + | 72 | ( | x | + | 21 | ) |
| = | 72 | x | − | 1512 | + | 72 | x | + | 72 | × | 21 |
| = | 72 | x | − | 1512 | + | 72 | x | + | 1512 |
| = | 144 | x | − | 0 |
| 144 | x | − | 0 | = | 7 |
| x | = | 7 | ÷ | 144 |
| = | 7 | × | 1 144 |
| x | = | 7 144 |
| x | = | 0.048611 |