| ( | a | − | 20 | ) | ( | 340 | − | 10 | a | ) | = | 400 |
| Left side of the equation = | a | ( | 340 | − | 10 | a | ) | − | 20 | ( | 340 | − | 10 | a | ) |
| = | a | × | 340 | − | a | × | 10 | a | − | 20 | ( | 340 | − | 10 | a | ) |
| = | 340 | a | − | a | × | 10 | a | − | 20 | × | 340 | + | 20 | × | 10 | a |
| = | 340 | a | − | a | × | 10 | a | − | 6800 | + | 200 | a |
| = | 540 | a | − | a | × | 10 | a | − | 6800 |
| 540 | a | − | a | × | 10 | a | − | 6800 | = | 400 |