| ( | 10 | x | + | 300 | ) | ( | 400 | − | x | − | 300 | ) | = | 40000 |
| Left side of the equation = | 10 | x | ( | 400 | − | x | − | 300 | ) | + | 300 | ( | 400 | − | x | − | 300 | ) |
| = | 10 | x | × | 400 | − | 10 | x | x | − | 10 | x | × | 300 | + | 300 | ( | 400 | − | x | − | 300 | ) |
| = | 4000 | x | − | 10 | x | x | − | 3000 | x | + | 300 | ( | 400 | − | x | − | 300 | ) |
| = | 1000 | x | − | 10 | x | x | + | 300 | ( | 400 | − | x | − | 300 | ) |
| = | 1000 | x | − | 10 | x | x | + | 300 | × | 400 | − | 300 | x | − | 300 | × | 300 |
| = | 1000 | x | − | 10 | x | x | + | 120000 | − | 300 | x | − | 90000 |
| = | 700 | x | − | 10 | x | x | + | 30000 |
| 700 | x | − | 10 | x | x | + | 30000 | = | 40000 |