| ( | 11 | − | 2 | x | ) | ( | 7 | − | 2 | x | ) | = | 21 |
| Left side of the equation = | 11 | ( | 7 | − | 2 | x | ) | − | 2 | x | ( | 7 | − | 2 | x | ) |
| = | 11 | × | 7 | − | 11 | × | 2 | x | − | 2 | x | ( | 7 | − | 2 | x | ) |
| = | 77 | − | 22 | x | − | 2 | x | ( | 7 | − | 2 | x | ) |
| = | 77 | − | 22 | x | − | 2 | x | × | 7 | + | 2 | x | × | 2 | x |
| = | 77 | − | 22 | x | − | 14 | x | + | 4 | x | x |
| = | 77 | − | 36 | x | + | 4 | x | x |
| 77 | − | 36 | x | + | 4 | x | x | = | 21 |