| ( | x | − | 30 | ) | ( | 280 | − | 10 | x | + | 400 | ) | = | 3250 |
| Left side of the equation = | x | ( | 280 | − | 10 | x | + | 400 | ) | − | 30 | ( | 280 | − | 10 | x | + | 400 | ) |
| = | x | × | 280 | − | x | × | 10 | x | + | x | × | 400 | − | 30 | ( | 280 | − | 10 | x | + | 400 | ) |
| = | 680 | x | − | x | × | 10 | x | − | 30 | ( | 280 | − | 10 | x | + | 400 | ) |
| = | 680 | x | − | x | × | 10 | x | − | 30 | × | 280 | + | 30 | × | 10 | x | − | 30 | × | 400 |
| = | 680 | x | − | x | × | 10 | x | − | 8400 | + | 300 | x | − | 12000 |
| = | 980 | x | − | x | × | 10 | x | − | 20400 |
| 980 | x | − | x | × | 10 | x | − | 20400 | = | 3250 |