| ( | x | − | 12 | ) | ( | 510 | − | 10 | x | + | 200 | ) | = | 7900 |
| Left side of the equation = | x | ( | 510 | − | 10 | x | + | 200 | ) | − | 12 | ( | 510 | − | 10 | x | + | 200 | ) |
| = | x | × | 510 | − | x | × | 10 | x | + | x | × | 200 | − | 12 | ( | 510 | − | 10 | x | + | 200 | ) |
| = | 710 | x | − | x | × | 10 | x | − | 12 | ( | 510 | − | 10 | x | + | 200 | ) |
| = | 710 | x | − | x | × | 10 | x | − | 12 | × | 510 | + | 12 | × | 10 | x | − | 12 | × | 200 |
| = | 710 | x | − | x | × | 10 | x | − | 6120 | + | 120 | x | − | 2400 |
| = | 830 | x | − | x | × | 10 | x | − | 8520 |
| 830 | x | − | x | × | 10 | x | − | 8520 | = | 7900 |