| ( | 7 | − | x | ) | ( | 300 | + | 20 | x | ) | = | 3000 |
| Left side of the equation = | 7 | ( | 300 | + | 20 | x | ) | − | x | ( | 300 | + | 20 | x | ) |
| = | 7 | × | 300 | + | 7 | × | 20 | x | − | x | ( | 300 | + | 20 | x | ) |
| = | 2100 | + | 140 | x | − | x | ( | 300 | + | 20 | x | ) |
| = | 2100 | + | 140 | x | − | x | × | 300 | − | x | × | 20 | x |
| = | 2100 | − | 160 | x | − | x | × | 20 | x |
| 2100 | − | 160 | x | − | x | × | 20 | x | = | 3000 |