| ( | 10 | − | x | ) | ( | 40 | + | 10 | x | ) | = | 480 |
| Left side of the equation = | 10 | ( | 40 | + | 10 | x | ) | − | x | ( | 40 | + | 10 | x | ) |
| = | 10 | × | 40 | + | 10 | × | 10 | x | − | x | ( | 40 | + | 10 | x | ) |
| = | 400 | + | 100 | x | − | x | ( | 40 | + | 10 | x | ) |
| = | 400 | + | 100 | x | − | x | × | 40 | − | x | × | 10 | x |
| = | 400 | + | 60 | x | − | x | × | 10 | x |
| 400 | + | 60 | x | − | x | × | 10 | x | = | 480 |