| ( | x | − | 40 | ) | ( | - | 30 | x | + | 2100 | ) | = | 0 |
| Left side of the equation = | x | ( | - | 30 | x | + | 2100 | ) | − | 40 | ( | - | 30 | x | + | 2100 | ) |
| = | - | x | × | 30 | x | + | x | × | 2100 | − | 40 | ( | - | 30 | x | + | 2100 | ) |
| = | - | x | × | 30 | x | + | 2100 | x | + | 40 | × | 30 | x | − | 40 | × | 2100 |
| = | - | x | × | 30 | x | + | 2100 | x | + | 1200 | x | − | 84000 |
| = | - | x | × | 30 | x | + | 3300 | x | − | 84000 |
| - | x | × | 30 | x | + | 3300 | x | − | 84000 | = | 0 |