| ( | 1 | + | x | ) | ( | 900 | − | 30 | x | ) | = | 3900 |
| Left side of the equation = | 1 | ( | 900 | − | 30 | x | ) | + | x | ( | 900 | − | 30 | x | ) |
| = | 1 | × | 900 | − | 1 | × | 30 | x | + | x | ( | 900 | − | 30 | x | ) |
| = | 900 | − | 30 | x | + | x | ( | 900 | − | 30 | x | ) |
| = | 900 | − | 30 | x | + | x | × | 900 | − | x | × | 30 | x |
| = | 900 | + | 870 | x | − | x | × | 30 | x |
| 900 | + | 870 | x | − | x | × | 30 | x | = | 3900 |