| ( | 2 | + | x | ) | ( | 200 | − | 10 | x | ) | = | 640 |
| Left side of the equation = | 2 | ( | 200 | − | 10 | x | ) | + | x | ( | 200 | − | 10 | x | ) |
| = | 2 | × | 200 | − | 2 | × | 10 | x | + | x | ( | 200 | − | 10 | x | ) |
| = | 400 | − | 20 | x | + | x | ( | 200 | − | 10 | x | ) |
| = | 400 | − | 20 | x | + | x | × | 200 | − | x | × | 10 | x |
| = | 400 | + | 180 | x | − | x | × | 10 | x |
| 400 | + | 180 | x | − | x | × | 10 | x | = | 640 |