| ( | 200 | − | 5 | x | ) | ( | 2 | + | x | ) | = | 640 |
| Left side of the equation = | 200 | ( | 2 | + | x | ) | − | 5 | x | ( | 2 | + | x | ) |
| = | 200 | × | 2 | + | 200 | x | − | 5 | x | ( | 2 | + | x | ) |
| = | 400 | + | 200 | x | − | 5 | x | ( | 2 | + | x | ) |
| = | 400 | + | 200 | x | − | 5 | x | × | 2 | − | 5 | x | x |
| = | 400 | + | 200 | x | − | 10 | x | − | 5 | x | x |
| = | 400 | + | 190 | x | − | 5 | x | x |
| 400 | + | 190 | x | − | 5 | x | x | = | 640 |