| ( | 400 | − | x | ) | x | ( | 8 | + | 4 | x | ) | = | 0 |
| Left side of the equation = | 400 | x | ( | 8 | + | 4 | x | ) | − | x | x | ( | 8 | + | 4 | x | ) |
| = | 400 | x | × | 8 | + | 400 | x | × | 4 | x | − | x | x | ( | 8 | + | 4 | x | ) |
| = | 3200 | x | + | 1600 | x | x | − | x | x | ( | 8 | + | 4 | x | ) |
| = | 3200 | x | + | 1600 | x | x | − | x | x | × | 8 | − | x | x | × | 4 | x |
| 3200 | x | + | 1600 | x | x | − | x | x | × | 8 | − | x | x | × | 4 | x | = | 0 |