| 400 | = | ( | X | − | 40 | ) | ( | 140 | − | 2 | X | ) |
| Right side of the equation = | X | ( | 140 | − | 2 | X | ) | − | 40 | ( | 140 | − | 2 | X | ) |
| = | X | × | 140 | − | X | × | 2 | X | − | 40 | ( | 140 | − | 2 | X | ) |
| = | 140 | X | − | X | × | 2 | X | − | 40 | × | 140 | + | 40 | × | 2 | X |
| = | 140 | X | − | X | × | 2 | X | − | 5600 | + | 80 | X |
| = | 220 | X | − | X | × | 2 | X | − | 5600 |
| 400 | = | 220 | X | − | X | × | 2 | X | − | 5600 |