| ( | x | − | 3 10 | ) | ( | 7 20 | − | x | ) | = | 31 50 | x |
| Left side of the equation = | x | ( | 7 20 | − | x | ) | − | 3 10 | ( | 7 20 | − | x | ) |
| = | x | × | 7 20 | − | x | x | − | 3 10 | ( | 7 20 | − | x | ) |
| = | 7 20 | x | − | x | x | − | 3 10 | × | 7 20 | + | 3 10 | x |
| = | 7 20 | x | − | x | x | − | 21 200 | + | 3 10 | x |
| = | 13 20 | x | − | x | x | − | 21 200 |
13 20 | x | − | x | x | − | 21 200 | = | 31 50 | x |