( | 10 | − | a | ) | ( | 15 | a | + | 120 | ) | = | 840 |
Left side of the equation = | 10 | ( | 15 | a | + | 120 | ) | − | a | ( | 15 | a | + | 120 | ) |
= | 10 | × | 15 | a | + | 10 | × | 120 | − | a | ( | 15 | a | + | 120 | ) |
= | 150 | a | + | 1200 | − | a | ( | 15 | a | + | 120 | ) |
= | 150 | a | + | 1200 | − | a | × | 15 | a | − | a | × | 120 |
= | 30 | a | + | 1200 | − | a | × | 15 | a |
30 | a | + | 1200 | − | a | × | 15 | a | = | 840 |