| ( | 800 | − | a | ) | × | 9 | + | 3000 | = | ( | 800 | − | a | ) | × | 8 | + | 3500 |
| Left side of the equation = | 800 | × | 9 | − | a | × | 9 | + | 3000 |
| = | 7200 | − | a | × | 9 | + | 3000 |
| = | 10200 | − | 9 | a |
| 10200 | − | 9 | a | = | ( | 800 | − | a | ) | × | 8 | + | 3500 |
| Right side of the equation = | 800 | × | 8 | − | a | × | 8 | + | 3500 |
| = | 6400 | − | a | × | 8 | + | 3500 |
| = | 9900 | − | 8 | a |
| 10200 | − | 9 | a | = | 9900 | − | 8 | a |
| - | 9 | a | + | 8 | a | = | 9900 | − | 10200 |
| - | a | = | 9900 | − | 10200 |
| - | a | = | 9900 | − | 10200 |
| - | a | = | - | 300 |
| 300 | = | a |
| a | = | 300 |