| ( | 300 | − | a | ) | ( | 80 | + | 2 | a | ) | = | 3600 |
| Left side of the equation = | 300 | ( | 80 | + | 2 | a | ) | − | a | ( | 80 | + | 2 | a | ) |
| = | 300 | × | 80 | + | 300 | × | 2 | a | − | a | ( | 80 | + | 2 | a | ) |
| = | 24000 | + | 600 | a | − | a | ( | 80 | + | 2 | a | ) |
| = | 24000 | + | 600 | a | − | a | × | 80 | − | a | × | 2 | a |
| = | 24000 | + | 520 | a | − | a | × | 2 | a |
| 24000 | + | 520 | a | − | a | × | 2 | a | = | 3600 |