| ( | 1 | − | m | ) | ( | 300 | + | 10 | m | ) | = | 420 |
| Left side of the equation = | 1 | ( | 300 | + | 10 | m | ) | − | m | ( | 300 | + | 10 | m | ) |
| = | 1 | × | 300 | + | 1 | × | 10 | m | − | m | ( | 300 | + | 10 | m | ) |
| = | 300 | + | 10 | m | − | m | ( | 300 | + | 10 | m | ) |
| = | 300 | + | 10 | m | − | m | × | 300 | − | m | × | 10 | m |
| = | 300 | − | 290 | m | − | m | × | 10 | m |
| 300 | − | 290 | m | − | m | × | 10 | m | = | 420 |