| 2 | ( | 21 | + | 2 | x | ) | ( | 15 | + | x | ) | + | 2 | x | = | 875 |
| 方程左边 = | 2 | × | 21 | ( | 15 | + | x | ) | + | 2 | × | 2 | x | ( | 15 | + | x | ) | + | 2 | x |
| = | 42 | ( | 15 | + | x | ) | + | 4 | x | ( | 15 | + | x | ) | + | 2 | x |
| = | 42 | × | 15 | + | 42 | x | + | 4 | x | ( | 15 | + | x | ) | + | 2 | x |
| = | 630 | + | 42 | x | + | 4 | x | ( | 15 | + | x | ) | + | 2 | x |
| = | 630 | + | 44 | x | + | 4 | x | ( | 15 | + | x | ) |
| = | 630 | + | 44 | x | + | 4 | x | × | 15 | + | 4 | x | x |
| = | 630 | + | 44 | x | + | 60 | x | + | 4 | x | x |
| = | 630 | + | 104 | x | + | 4 | x | x |
| 630 | + | 104 | x | + | 4 | x | x | = | 875 |