| ( | 40 | + | 10 | x | ) | ( | 4 | − | x | ) | = | 0 |
| Left side of the equation = | 40 | ( | 4 | − | x | ) | + | 10 | x | ( | 4 | − | x | ) |
| = | 40 | × | 4 | − | 40 | x | + | 10 | x | ( | 4 | − | x | ) |
| = | 160 | − | 40 | x | + | 10 | x | ( | 4 | − | x | ) |
| = | 160 | − | 40 | x | + | 10 | x | × | 4 | − | 10 | x | x |
| = | 160 | − | 40 | x | + | 40 | x | − | 10 | x | x |
| = | 160 | − | 0 | x | − | 10 | x | x |
| 160 | − | 0 | x | − | 10 | x | x | = | 0 |