| ( | 12 | − | 2 | x | ) | ( | 7 | − | 2 | x | ) | = | 21 |
| Left side of the equation = | 12 | ( | 7 | − | 2 | x | ) | − | 2 | x | ( | 7 | − | 2 | x | ) |
| = | 12 | × | 7 | − | 12 | × | 2 | x | − | 2 | x | ( | 7 | − | 2 | x | ) |
| = | 84 | − | 24 | x | − | 2 | x | ( | 7 | − | 2 | x | ) |
| = | 84 | − | 24 | x | − | 2 | x | × | 7 | + | 2 | x | × | 2 | x |
| = | 84 | − | 24 | x | − | 14 | x | + | 4 | x | x |
| = | 84 | − | 38 | x | + | 4 | x | x |
| 84 | − | 38 | x | + | 4 | x | x | = | 21 |