| ( | 120 | − | x | ) | ( | 400 | + | 3 | x | ) | = | 13350 |
| Left side of the equation = | 120 | ( | 400 | + | 3 | x | ) | − | x | ( | 400 | + | 3 | x | ) |
| = | 120 | × | 400 | + | 120 | × | 3 | x | − | x | ( | 400 | + | 3 | x | ) |
| = | 48000 | + | 360 | x | − | x | ( | 400 | + | 3 | x | ) |
| = | 48000 | + | 360 | x | − | x | × | 400 | − | x | × | 3 | x |
| = | 48000 | − | 40 | x | − | x | × | 3 | x |
| 48000 | − | 40 | x | − | x | × | 3 | x | = | 13350 |