| ( | 30 | − | a | ) | ( | 180 | + | 10 | a | ) | = | 3200 |
| Left side of the equation = | 30 | ( | 180 | + | 10 | a | ) | − | a | ( | 180 | + | 10 | a | ) |
| = | 30 | × | 180 | + | 30 | × | 10 | a | − | a | ( | 180 | + | 10 | a | ) |
| = | 5400 | + | 300 | a | − | a | ( | 180 | + | 10 | a | ) |
| = | 5400 | + | 300 | a | − | a | × | 180 | − | a | × | 10 | a |
| = | 5400 | + | 120 | a | − | a | × | 10 | a |
| 5400 | + | 120 | a | − | a | × | 10 | a | = | 3200 |