| ( | 40 | − | x | ) | ( | 10 | x | + | 200 | ) | = | 8910 |
| Left side of the equation = | 40 | ( | 10 | x | + | 200 | ) | − | x | ( | 10 | x | + | 200 | ) |
| = | 40 | × | 10 | x | + | 40 | × | 200 | − | x | ( | 10 | x | + | 200 | ) |
| = | 400 | x | + | 8000 | − | x | ( | 10 | x | + | 200 | ) |
| = | 400 | x | + | 8000 | − | x | × | 10 | x | − | x | × | 200 |
| = | 200 | x | + | 8000 | − | x | × | 10 | x |
| 200 | x | + | 8000 | − | x | × | 10 | x | = | 8910 |