| ( | m | − | 20 | ) | ( | - | 3 | m | + | 300 | ) | = | 2100 |
| Left side of the equation = | m | ( | - | 3 | m | + | 300 | ) | − | 20 | ( | - | 3 | m | + | 300 | ) |
| = | - | m | × | 3 | m | + | m | × | 300 | − | 20 | ( | - | 3 | m | + | 300 | ) |
| = | - | m | × | 3 | m | + | 300 | m | + | 20 | × | 3 | m | − | 20 | × | 300 |
| = | - | m | × | 3 | m | + | 300 | m | + | 60 | m | − | 6000 |
| = | - | m | × | 3 | m | + | 360 | m | − | 6000 |
| - | m | × | 3 | m | + | 360 | m | − | 6000 | = | 2100 |