| ( | 10 | x | + | 90 | ) | ( | x | − | 4 | ) | = | 1620 |
| Left side of the equation = | 10 | x | ( | x | − | 4 | ) | + | 90 | ( | x | − | 4 | ) |
| = | 10 | x | x | − | 10 | x | × | 4 | + | 90 | ( | x | − | 4 | ) |
| = | 10 | x | x | − | 40 | x | + | 90 | ( | x | − | 4 | ) |
| = | 10 | x | x | − | 40 | x | + | 90 | x | − | 90 | × | 4 |
| = | 10 | x | x | − | 40 | x | + | 90 | x | − | 360 |
| = | 10 | x | x | + | 50 | x | − | 360 |
| 10 | x | x | + | 50 | x | − | 360 | = | 1620 |