| ( | 100 | + | x | ) | ( | 100 | − | 9 | x | ) | = | 3000 |
| Left side of the equation = | 100 | ( | 100 | − | 9 | x | ) | + | x | ( | 100 | − | 9 | x | ) |
| = | 100 | × | 100 | − | 100 | × | 9 | x | + | x | ( | 100 | − | 9 | x | ) |
| = | 10000 | − | 900 | x | + | x | ( | 100 | − | 9 | x | ) |
| = | 10000 | − | 900 | x | + | x | × | 100 | − | x | × | 9 | x |
| = | 10000 | − | 800 | x | − | x | × | 9 | x |
| 10000 | − | 800 | x | − | x | × | 9 | x | = | 3000 |