| ( | 20 | − | a | ) | ( | 30 | a | + | 300 | ) | = | 6480 |
| Left side of the equation = | 20 | ( | 30 | a | + | 300 | ) | − | a | ( | 30 | a | + | 300 | ) |
| = | 20 | × | 30 | a | + | 20 | × | 300 | − | a | ( | 30 | a | + | 300 | ) |
| = | 600 | a | + | 6000 | − | a | ( | 30 | a | + | 300 | ) |
| = | 600 | a | + | 6000 | − | a | × | 30 | a | − | a | × | 300 |
| = | 300 | a | + | 6000 | − | a | × | 30 | a |
| 300 | a | + | 6000 | − | a | × | 30 | a | = | 6480 |