| ( | 13 | − | x | ) | ( | 180 | + | 4 | x | ) | = | 1920 |
| Left side of the equation = | 13 | ( | 180 | + | 4 | x | ) | − | x | ( | 180 | + | 4 | x | ) |
| = | 13 | × | 180 | + | 13 | × | 4 | x | − | x | ( | 180 | + | 4 | x | ) |
| = | 2340 | + | 52 | x | − | x | ( | 180 | + | 4 | x | ) |
| = | 2340 | + | 52 | x | − | x | × | 180 | − | x | × | 4 | x |
| = | 2340 | − | 128 | x | − | x | × | 4 | x |
| 2340 | − | 128 | x | − | x | × | 4 | x | = | 1920 |