| ( | 6 | + | x | ) | ( | 800 | − | 20 | x | ) | = | 0 |
| Left side of the equation = | 6 | ( | 800 | − | 20 | x | ) | + | x | ( | 800 | − | 20 | x | ) |
| = | 6 | × | 800 | − | 6 | × | 20 | x | + | x | ( | 800 | − | 20 | x | ) |
| = | 4800 | − | 120 | x | + | x | ( | 800 | − | 20 | x | ) |
| = | 4800 | − | 120 | x | + | x | × | 800 | − | x | × | 20 | x |
| = | 4800 | + | 680 | x | − | x | × | 20 | x |
| 4800 | + | 680 | x | − | x | × | 20 | x | = | 0 |