| 800 | + | ( | 1 | + | x | ) | ( | 80 | − | 2 | x | ) | − | 1200 | = | 0 |
| Left side of the equation = | - | 400 | + | ( | 1 | + | x | ) | ( | 80 | − | 2 | x | ) |
| - | 400 | + | ( | 1 | + | x | ) | ( | 80 | − | 2 | x | ) | = | 0 |
| Left side of the equation = | - | 400 | + | 1 | ( | 80 | − | 2 | x | ) | + | x | ( | 80 | − | 2 | x | ) |
| = | - | 400 | + | 1 | × | 80 | − | 1 | × | 2 | x | + | x | ( | 80 | − | 2 | x | ) |
| = | - | 400 | + | 80 | − | 2 | x | + | x | ( | 80 | − | 2 | x | ) |
| = | - | 320 | − | 2 | x | + | x | ( | 80 | − | 2 | x | ) |
| = | - | 320 | − | 2 | x | + | x | × | 80 | − | x | × | 2 | x |
| = | - | 320 | + | 78 | x | − | x | × | 2 | x |
| - | 320 | + | 78 | x | − | x | × | 2 | x | = | 0 |