| ( | 300 | − | 5 | x | ) | ( | 4 | + | x | ) | = | 1680 |
| Left side of the equation = | 300 | ( | 4 | + | x | ) | − | 5 | x | ( | 4 | + | x | ) |
| = | 300 | × | 4 | + | 300 | x | − | 5 | x | ( | 4 | + | x | ) |
| = | 1200 | + | 300 | x | − | 5 | x | ( | 4 | + | x | ) |
| = | 1200 | + | 300 | x | − | 5 | x | × | 4 | − | 5 | x | x |
| = | 1200 | + | 300 | x | − | 20 | x | − | 5 | x | x |
| = | 1200 | + | 280 | x | − | 5 | x | x |
| 1200 | + | 280 | x | − | 5 | x | x | = | 1680 |