| ( | 90 | − | 2 | x | ) | ( | 60 | − | 2 | x | ) | = | 4200 |
| Left side of the equation = | 90 | ( | 60 | − | 2 | x | ) | − | 2 | x | ( | 60 | − | 2 | x | ) |
| = | 90 | × | 60 | − | 90 | × | 2 | x | − | 2 | x | ( | 60 | − | 2 | x | ) |
| = | 5400 | − | 180 | x | − | 2 | x | ( | 60 | − | 2 | x | ) |
| = | 5400 | − | 180 | x | − | 2 | x | × | 60 | + | 2 | x | × | 2 | x |
| = | 5400 | − | 180 | x | − | 120 | x | + | 4 | x | x |
| = | 5400 | − | 300 | x | + | 4 | x | x |
| 5400 | − | 300 | x | + | 4 | x | x | = | 4200 |