| ( | 12 | − | x | ) | ( | 100 | + | 20 | x | ) | = | 1400 |
| Left side of the equation = | 12 | ( | 100 | + | 20 | x | ) | − | x | ( | 100 | + | 20 | x | ) |
| = | 12 | × | 100 | + | 12 | × | 20 | x | − | x | ( | 100 | + | 20 | x | ) |
| = | 1200 | + | 240 | x | − | x | ( | 100 | + | 20 | x | ) |
| = | 1200 | + | 240 | x | − | x | × | 100 | − | x | × | 20 | x |
| = | 1200 | + | 140 | x | − | x | × | 20 | x |
| 1200 | + | 140 | x | − | x | × | 20 | x | = | 1400 |