| ( | 1400 | − | x | ) | ( | 700 | − | x | ) | − | 800 | ( | 400 | − | x | ) | = | 0 |
| Left side of the equation = | 1400 | ( | 700 | − | x | ) | − | x | ( | 700 | − | x | ) | − | 800 | ( | 400 | − | x | ) |
| = | 1400 | × | 700 | − | 1400 | x | − | x | ( | 700 | − | x | ) | − | 800 | ( | 400 | − | x | ) |
| = | 980000 | − | 1400 | x | − | x | ( | 700 | − | x | ) | − | 800 | ( | 400 | − | x | ) |
| = | 980000 | − | 1400 | x | − | x | × | 700 | + | x | x | − | 800 | ( | 400 | − | x | ) |
| = | 980000 | − | 2100 | x | + | x | x | − | 800 | ( | 400 | − | x | ) |
| = | 980000 | − | 2100 | x | + | x | x | − | 800 | × | 400 | + | 800 | x |
| = | 980000 | − | 2100 | x | + | x | x | − | 320000 | + | 800 | x |
| = | 660000 | − | 1300 | x | + | x | x |
| 660000 | − | 1300 | x | + | x | x | = | 0 |