| ( | x | + | 7 | ) | ( | x | + | 8 | ) | ( | x | + | 9 | ) | = | 990 |
| Left side of the equation = | x | ( | x | + | 8 | ) | ( | x | + | 9 | ) | + | 7 | ( | x | + | 8 | ) | ( | x | + | 9 | ) |
| = | x | x | ( | x | + | 9 | ) | + | x | × | 8 | ( | x | + | 9 | ) | + | 7 | ( | x | + | 8 | ) | ( | x | + | 9 | ) |
| = | x | x | x | + | x | x | × | 9 | + | x | × | 8 | ( | x | + | 9 | ) | + | 7 | ( | x | + | 8 | ) | ( | x | + | 9 | ) |
| = | x | x | x | + | x | x | × | 9 | + | x | × | 8 | x | + | x | × | 8 | × | 9 |
| = | x | x | x | + | x | x | × | 9 | + | x | × | 8 | x | + | x | × | 72 | + | 7 |
| = | x | x | x | + | x | x | × | 9 | + | x | × | 8 | x | + | 72 | x | + | 7 |
| = | x | x | x | + | x | x | × | 9 | + | x | × | 8 | x | + | 72 | x | + | 7 |
| = | x | x | x | + | x | x | × | 9 | + | x | × | 8 | x | + | 72 | x | + | 7 |
| = | x | x | x | + | x | x | × | 9 | + | x | × | 8 | x | + | 72 | x | + | 7 |
| = | x | x | x | + | x | x | × | 9 | + | x | × | 8 | x | + | 135 | x | + | 7 |
| = | x | x | x | + | x | x | × | 9 | + | x | × | 8 | x | + | 135 | x | + | 7 |
| = | x | x | x | + | x | x | × | 9 | + | x | × | 8 | x | + | 135 | x | + | 7 |
| = | x | x | x | + | x | x | × | 9 | + | x | × | 8 | x | + | 191 | x | + | 7 |
| x | x | x | + | x | x | × | 9 | + | x | × | 8 | x | + | 191 | x | + | 7 | = | 990 |