| ( | 21 | − | 3 | x | ) | ( | 10 | − | 2 | x | ) | = | 0 |
| 方程左边 = | 21 | ( | 10 | − | 2 | x | ) | − | 3 | x | ( | 10 | − | 2 | x | ) |
| = | 21 | × | 10 | − | 21 | × | 2 | x | − | 3 | x | ( | 10 | − | 2 | x | ) |
| = | 210 | − | 42 | x | − | 3 | x | ( | 10 | − | 2 | x | ) |
| = | 210 | − | 42 | x | − | 3 | x | × | 10 | + | 3 | x | × | 2 | x |
| = | 210 | − | 42 | x | − | 30 | x | + | 6 | x | x |
| = | 210 | − | 72 | x | + | 6 | x | x |
| 210 | − | 72 | x | + | 6 | x | x | = | 0 |