| ( | 20 | − | x | ) | ( | - | 30 | x | + | 2100 | ) | = | 0 |
| Left side of the equation = | 20 | ( | - | 30 | x | + | 2100 | ) | − | x | ( | - | 30 | x | + | 2100 | ) |
| = | - | 20 | × | 30 | x | + | 20 | × | 2100 | − | x | ( | - | 30 | x | + | 2100 | ) |
| = | - | 600 | x | + | 42000 | − | x | ( | - | 30 | x | + | 2100 | ) |
| = | - | 600 | x | + | 42000 | + | x | × | 30 | x | − | x | × | 2100 |
| = | - | 2700 | x | + | 42000 | + | x | × | 30 | x |
| - | 2700 | x | + | 42000 | + | x | × | 30 | x | = | 0 |