| ( | x | − | 40 | ) | ( | 10 | x | + | 100 | ) | = | 2090 |
| Left side of the equation = | x | ( | 10 | x | + | 100 | ) | − | 40 | ( | 10 | x | + | 100 | ) |
| = | x | × | 10 | x | + | x | × | 100 | − | 40 | ( | 10 | x | + | 100 | ) |
| = | x | × | 10 | x | + | 100 | x | − | 40 | × | 10 | x | − | 40 | × | 100 |
| = | x | × | 10 | x | + | 100 | x | − | 400 | x | − | 4000 |
| = | x | × | 10 | x | − | 300 | x | − | 4000 |
| x | × | 10 | x | − | 300 | x | − | 4000 | = | 2090 |