| ( | 20 | − | x | ) | ( | 400 | + | 4 | x | ) | = | 6400 |
| Left side of the equation = | 20 | ( | 400 | + | 4 | x | ) | − | x | ( | 400 | + | 4 | x | ) |
| = | 20 | × | 400 | + | 20 | × | 4 | x | − | x | ( | 400 | + | 4 | x | ) |
| = | 8000 | + | 80 | x | − | x | ( | 400 | + | 4 | x | ) |
| = | 8000 | + | 80 | x | − | x | × | 400 | − | x | × | 4 | x |
| = | 8000 | − | 320 | x | − | x | × | 4 | x |
| 8000 | − | 320 | x | − | x | × | 4 | x | = | 6400 |