| ( | 12 | − | x | ) | ( | 200 | + | 40 | x | ) | = | 2800 |
| Left side of the equation = | 12 | ( | 200 | + | 40 | x | ) | − | x | ( | 200 | + | 40 | x | ) |
| = | 12 | × | 200 | + | 12 | × | 40 | x | − | x | ( | 200 | + | 40 | x | ) |
| = | 2400 | + | 480 | x | − | x | ( | 200 | + | 40 | x | ) |
| = | 2400 | + | 480 | x | − | x | × | 200 | − | x | × | 40 | x |
| = | 2400 | + | 280 | x | − | x | × | 40 | x |
| 2400 | + | 280 | x | − | x | × | 40 | x | = | 2800 |