| ( | 80 | + | x | ) | ( | 300 | − | 10 | x | ) | = | 0 |
| Left side of the equation = | 80 | ( | 300 | − | 10 | x | ) | + | x | ( | 300 | − | 10 | x | ) |
| = | 80 | × | 300 | − | 80 | × | 10 | x | + | x | ( | 300 | − | 10 | x | ) |
| = | 24000 | − | 800 | x | + | x | ( | 300 | − | 10 | x | ) |
| = | 24000 | − | 800 | x | + | x | × | 300 | − | x | × | 10 | x |
| = | 24000 | − | 500 | x | − | x | × | 10 | x |
| 24000 | − | 500 | x | − | x | × | 10 | x | = | 0 |