| ( | a | − | 21 | ) | ( | 10 | a | − | 35 | ) | − | 400 | = | 0 |
| Left side of the equation = | a | ( | 10 | a | − | 35 | ) | − | 21 | ( | 10 | a | − | 35 | ) | − | 400 |
| = | a | × | 10 | a | − | a | × | 35 | − | 21 | ( | 10 | a | − | 35 | ) | − | 400 |
| = | a | × | 10 | a | − | 35 | a | − | 21 | × | 10 | a | + | 21 | × | 35 | − | 400 |
| = | a | × | 10 | a | − | 35 | a | − | 210 | a | + | 735 | − | 400 |
| = | a | × | 10 | a | − | 245 | a | + | 335 |
| a | × | 10 | a | − | 245 | a | + | 335 | = | 0 |