| ( | 90 | − | 3 | x | ) | ( | 60 | − | 3 | x | ) | = | 0 |
| Left side of the equation = | 90 | ( | 60 | − | 3 | x | ) | − | 3 | x | ( | 60 | − | 3 | x | ) |
| = | 90 | × | 60 | − | 90 | × | 3 | x | − | 3 | x | ( | 60 | − | 3 | x | ) |
| = | 5400 | − | 270 | x | − | 3 | x | ( | 60 | − | 3 | x | ) |
| = | 5400 | − | 270 | x | − | 3 | x | × | 60 | + | 3 | x | × | 3 | x |
| = | 5400 | − | 270 | x | − | 180 | x | + | 9 | x | x |
| = | 5400 | − | 450 | x | + | 9 | x | x |
| 5400 | − | 450 | x | + | 9 | x | x | = | 0 |