| ( | 200 | − | a | ) | ( | 20 | − | 2 | a | ) | = | 1150 |
| Left side of the equation = | 200 | ( | 20 | − | 2 | a | ) | − | a | ( | 20 | − | 2 | a | ) |
| = | 200 | × | 20 | − | 200 | × | 2 | a | − | a | ( | 20 | − | 2 | a | ) |
| = | 4000 | − | 400 | a | − | a | ( | 20 | − | 2 | a | ) |
| = | 4000 | − | 400 | a | − | a | × | 20 | + | a | × | 2 | a |
| = | 4000 | − | 420 | a | + | a | × | 2 | a |
| 4000 | − | 420 | a | + | a | × | 2 | a | = | 1150 |