| ( | 15 | − | x | ) | ( | 400 | + | 40 | x | ) | = | 6240 |
| Left side of the equation = | 15 | ( | 400 | + | 40 | x | ) | − | x | ( | 400 | + | 40 | x | ) |
| = | 15 | × | 400 | + | 15 | × | 40 | x | − | x | ( | 400 | + | 40 | x | ) |
| = | 6000 | + | 600 | x | − | x | ( | 400 | + | 40 | x | ) |
| = | 6000 | + | 600 | x | − | x | × | 400 | − | x | × | 40 | x |
| = | 6000 | + | 200 | x | − | x | × | 40 | x |
| 6000 | + | 200 | x | − | x | × | 40 | x | = | 6240 |