| ( | a | − | 21 | ) | ( | 350 | − | 10 | a | ) | − | 400 | = | 0 |
| 方程左边 = | a | ( | 350 | − | 10 | a | ) | − | 21 | ( | 350 | − | 10 | a | ) | − | 400 |
| = | a | × | 350 | − | a | × | 10 | a | − | 21 | ( | 350 | − | 10 | a | ) | − | 400 |
| = | 350 | a | − | a | × | 10 | a | − | 21 | × | 350 | + | 21 | × | 10 | a | − | 400 |
| = | 350 | a | − | a | × | 10 | a | − | 7350 | + | 210 | a | − | 400 |
| = | 560 | a | − | a | × | 10 | a | − | 7750 |
| 560 | a | − | a | × | 10 | a | − | 7750 | = | 0 |