| ( | 35 | − | 2 | a | ) | ( | 20 | − | a | ) | = | 600 |
| Left side of the equation = | 35 | ( | 20 | − | a | ) | − | 2 | a | ( | 20 | − | a | ) |
| = | 35 | × | 20 | − | 35 | a | − | 2 | a | ( | 20 | − | a | ) |
| = | 700 | − | 35 | a | − | 2 | a | ( | 20 | − | a | ) |
| = | 700 | − | 35 | a | − | 2 | a | × | 20 | + | 2 | a | a |
| = | 700 | − | 35 | a | − | 40 | a | + | 2 | a | a |
| = | 700 | − | 75 | a | + | 2 | a | a |
| 700 | − | 75 | a | + | 2 | a | a | = | 600 |