| ( | 2 | − | a | ) | ( | 100 | + | 40 | a | ) | = | 300 |
| Left side of the equation = | 2 | ( | 100 | + | 40 | a | ) | − | a | ( | 100 | + | 40 | a | ) |
| = | 2 | × | 100 | + | 2 | × | 40 | a | − | a | ( | 100 | + | 40 | a | ) |
| = | 200 | + | 80 | a | − | a | ( | 100 | + | 40 | a | ) |
| = | 200 | + | 80 | a | − | a | × | 100 | − | a | × | 40 | a |
| = | 200 | − | 20 | a | − | a | × | 40 | a |
| 200 | − | 20 | a | − | a | × | 40 | a | = | 300 |