| ( | ( | 60 | − | a | ) | − | 40 | ) | ( | 300 | + | 3 | a | ) | = | 6480 |
| Left side of the equation = | ( | 60 | − | a | ) | ( | 300 | + | 3 | a | ) | − | 40 | ( | 300 | + | 3 | a | ) |
| = | 60 | ( | 300 | + | 3 | a | ) | − | a | ( | 300 | + | 3 | a | ) | − | 40 | ( | 300 | + | 3 | a | ) |
| = | 60 | × | 300 | + | 60 | × | 3 | a | − | a | ( | 300 | + | 3 | a | ) | − | 40 | ( | 300 | + | 3 | a | ) |
| = | 18000 | + | 180 | a | − | a | ( | 300 | + | 3 | a | ) | − | 40 | ( | 300 | + | 3 | a | ) |
| = | 18000 | + | 180 | a | − | a | × | 300 | − | a | × | 3 | a | − | 40 | ( | 300 | + | 3 | a | ) |
| = | 18000 | − | 120 | a | − | a | × | 3 | a | − | 40 | ( | 300 | + | 3 | a | ) |
| = | 18000 | − | 120 | a | − | a | × | 3 | a | − | 40 | × | 300 | − | 40 | × | 3 | a |
| = | 18000 | − | 120 | a | − | a | × | 3 | a | − | 12000 | − | 120 | a |
| = | 6000 | − | 240 | a | − | a | × | 3 | a |
| 6000 | − | 240 | a | − | a | × | 3 | a | = | 6480 |