| ( | 50 | − | x | ) | ( | 180 | + | 10 | x | ) | = | 9870 |
| Left side of the equation = | 50 | ( | 180 | + | 10 | x | ) | − | x | ( | 180 | + | 10 | x | ) |
| = | 50 | × | 180 | + | 50 | × | 10 | x | − | x | ( | 180 | + | 10 | x | ) |
| = | 9000 | + | 500 | x | − | x | ( | 180 | + | 10 | x | ) |
| = | 9000 | + | 500 | x | − | x | × | 180 | − | x | × | 10 | x |
| = | 9000 | + | 320 | x | − | x | × | 10 | x |
| 9000 | + | 320 | x | − | x | × | 10 | x | = | 9870 |