| ( | x | + | 2 | ) | ( | 200 | − | 20 | x | ) | = | 640 |
| Left side of the equation = | x | ( | 200 | − | 20 | x | ) | + | 2 | ( | 200 | − | 20 | x | ) |
| = | x | × | 200 | − | x | × | 20 | x | + | 2 | ( | 200 | − | 20 | x | ) |
| = | 200 | x | − | x | × | 20 | x | + | 2 | × | 200 | − | 2 | × | 20 | x |
| = | 200 | x | − | x | × | 20 | x | + | 400 | − | 40 | x |
| = | 160 | x | − | x | × | 20 | x | + | 400 |
| 160 | x | − | x | × | 20 | x | + | 400 | = | 640 |