| ( | 320 | − | 10 | a | ) | ( | a | − | 18 | ) | = | 400 |
| Left side of the equation = | 320 | ( | a | − | 18 | ) | − | 10 | a | ( | a | − | 18 | ) |
| = | 320 | a | − | 320 | × | 18 | − | 10 | a | ( | a | − | 18 | ) |
| = | 320 | a | − | 5760 | − | 10 | a | ( | a | − | 18 | ) |
| = | 320 | a | − | 5760 | − | 10 | a | a | + | 10 | a | × | 18 |
| = | 320 | a | − | 5760 | − | 10 | a | a | + | 180 | a |
| = | 500 | a | − | 5760 | − | 10 | a | a |
| 500 | a | − | 5760 | − | 10 | a | a | = | 400 |