| ( | 20 | − | 2 | x | ) | ( | 36 | − | 4 | x | ) | = | 144 |
| Left side of the equation = | 20 | ( | 36 | − | 4 | x | ) | − | 2 | x | ( | 36 | − | 4 | x | ) |
| = | 20 | × | 36 | − | 20 | × | 4 | x | − | 2 | x | ( | 36 | − | 4 | x | ) |
| = | 720 | − | 80 | x | − | 2 | x | ( | 36 | − | 4 | x | ) |
| = | 720 | − | 80 | x | − | 2 | x | × | 36 | + | 2 | x | × | 4 | x |
| = | 720 | − | 80 | x | − | 72 | x | + | 8 | x | x |
| = | 720 | − | 152 | x | + | 8 | x | x |
| 720 | − | 152 | x | + | 8 | x | x | = | 144 |