| ( | 40 | − | 3 | x | ) | ( | 15 | − | x | ) | = | 198 |
| Left side of the equation = | 40 | ( | 15 | − | x | ) | − | 3 | x | ( | 15 | − | x | ) |
| = | 40 | × | 15 | − | 40 | x | − | 3 | x | ( | 15 | − | x | ) |
| = | 600 | − | 40 | x | − | 3 | x | ( | 15 | − | x | ) |
| = | 600 | − | 40 | x | − | 3 | x | × | 15 | + | 3 | x | x |
| = | 600 | − | 40 | x | − | 45 | x | + | 3 | x | x |
| = | 600 | − | 85 | x | + | 3 | x | x |
| 600 | − | 85 | x | + | 3 | x | x | = | 198 |
| x2= | 67 3 |