| ( | 80 | − | x | ) | ( | 20 | + | 2 | x | ) | = | 2800 |
| Left side of the equation = | 80 | ( | 20 | + | 2 | x | ) | − | x | ( | 20 | + | 2 | x | ) |
| = | 80 | × | 20 | + | 80 | × | 2 | x | − | x | ( | 20 | + | 2 | x | ) |
| = | 1600 | + | 160 | x | − | x | ( | 20 | + | 2 | x | ) |
| = | 1600 | + | 160 | x | − | x | × | 20 | − | x | × | 2 | x |
| = | 1600 | + | 140 | x | − | x | × | 2 | x |
| 1600 | + | 140 | x | − | x | × | 2 | x | = | 2800 |