| ( | x | − | 21 | ) | ( | 350 | − | 10 | x | ) | = | 400 |
| Left side of the equation = | x | ( | 350 | − | 10 | x | ) | − | 21 | ( | 350 | − | 10 | x | ) |
| = | x | × | 350 | − | x | × | 10 | x | − | 21 | ( | 350 | − | 10 | x | ) |
| = | 350 | x | − | x | × | 10 | x | − | 21 | × | 350 | + | 21 | × | 10 | x |
| = | 350 | x | − | x | × | 10 | x | − | 7350 | + | 210 | x |
| = | 560 | x | − | x | × | 10 | x | − | 7350 |
| 560 | x | − | x | × | 10 | x | − | 7350 | = | 400 |