| 900 | − | 30 | − | ( | 30 | − | x | ) | ( | 29 | − | x | ) | = | 812 |
| Left side of the equation = | 870 | − | ( | 30 | − | x | ) | ( | 29 | − | x | ) |
| 870 | − | ( | 30 | − | x | ) | ( | 29 | − | x | ) | = | 812 |
| Left side of the equation = | 870 | − | 30 | ( | 29 | − | x | ) | + | x | ( | 29 | − | x | ) |
| = | 870 | − | 30 | × | 29 | + | 30 | x | + | x | ( | 29 | − | x | ) |
| = | 870 | − | 870 | + | 30 | x | + | x | ( | 29 | − | x | ) |
| = | 0 | + | 30 | x | + | x | ( | 29 | − | x | ) |
| = | 0 | + | 30 | x | + | x | × | 29 | − | x | x |
| = | 0 | + | 59 | x | − | x | x |
| 0 | + | 59 | x | − | x | x | = | 812 |