| ( | 50 | − | a | ) | ( | 50 | + | 5 | a | ) | = | 4000 |
| Left side of the equation = | 50 | ( | 50 | + | 5 | a | ) | − | a | ( | 50 | + | 5 | a | ) |
| = | 50 | × | 50 | + | 50 | × | 5 | a | − | a | ( | 50 | + | 5 | a | ) |
| = | 2500 | + | 250 | a | − | a | ( | 50 | + | 5 | a | ) |
| = | 2500 | + | 250 | a | − | a | × | 50 | − | a | × | 5 | a |
| = | 2500 | + | 200 | a | − | a | × | 5 | a |
| 2500 | + | 200 | a | − | a | × | 5 | a | = | 4000 |