| 2 | ( | x | + | 3 | ) | ( | 7 | − | x | ) | − | 4 | = | 56 |
| 方程左边 = | 2 | x | ( | 7 | − | x | ) | + | 2 | × | 3 | ( | 7 | − | x | ) | − | 4 |
| = | 2 | x | ( | 7 | − | x | ) | + | 6 | ( | 7 | − | x | ) | − | 4 |
| = | 2 | x | × | 7 | − | 2 | x | x | + | 6 | ( | 7 | − | x | ) | − | 4 |
| = | 14 | x | − | 2 | x | x | + | 6 | ( | 7 | − | x | ) | − | 4 |
| = | 14 | x | − | 2 | x | x | + | 6 | × | 7 | − | 6 | x | − | 4 |
| = | 14 | x | − | 2 | x | x | + | 42 | − | 6 | x | − | 4 |
| = | 8 | x | − | 2 | x | x | + | 38 |
| 8 | x | − | 2 | x | x | + | 38 | = | 56 |