| ( | 30 | − | x | ) | ( | 500 | + | 2 | x | ) | = | 10400 |
| Left side of the equation = | 30 | ( | 500 | + | 2 | x | ) | − | x | ( | 500 | + | 2 | x | ) |
| = | 30 | × | 500 | + | 30 | × | 2 | x | − | x | ( | 500 | + | 2 | x | ) |
| = | 15000 | + | 60 | x | − | x | ( | 500 | + | 2 | x | ) |
| = | 15000 | + | 60 | x | − | x | × | 500 | − | x | × | 2 | x |
| = | 15000 | − | 440 | x | − | x | × | 2 | x |
| 15000 | − | 440 | x | − | x | × | 2 | x | = | 10400 |