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