| ( | 500 | − | 100 | x | ) | ( | 3 | + | x | ) | = | 800 |
| Left side of the equation = | 500 | ( | 3 | + | x | ) | − | 100 | x | ( | 3 | + | x | ) |
| = | 500 | × | 3 | + | 500 | x | − | 100 | x | ( | 3 | + | x | ) |
| = | 1500 | + | 500 | x | − | 100 | x | ( | 3 | + | x | ) |
| = | 1500 | + | 500 | x | − | 100 | x | × | 3 | − | 100 | x | x |
| = | 1500 | + | 500 | x | − | 300 | x | − | 100 | x | x |
| = | 1500 | + | 200 | x | − | 100 | x | x |
| 1500 | + | 200 | x | − | 100 | x | x | = | 800 |