| ( | a | − | 21 | ) | ( | a | − | 35 | ) | − | 40 | + | 0 | = | 0 |
| Left side of the equation = | ( | a | − | 21 | ) | ( | a | − | 35 | ) | − | 40 |
| ( | a | − | 21 | ) | ( | a | − | 35 | ) | − | 40 | = | 0 |
| Left side of the equation = | a | ( | a | − | 35 | ) | − | 21 | ( | a | − | 35 | ) | − | 40 |
| = | a | a | − | a | × | 35 | − | 21 | ( | a | − | 35 | ) | − | 40 |
| = | a | a | − | 35 | a | − | 21 | a | + | 21 | × | 35 | − | 40 |
| = | a | a | − | 35 | a | − | 21 | a | + | 735 | − | 40 |
| = | a | a | − | 56 | a | + | 695 |
| a | a | − | 56 | a | + | 695 | = | 0 |