We want to solve for a, correct?
If that is so, then the left side is just standard subtracting fractions:
Since the fractions on the left side have a common denominator already, we can just combine them and write (a - 1)/2.
The right side, -6, remains the same.
Now, put it together:
(a - 1)/2 = -6
Multiply both sides by 2:
a - 1 = -12
Add both sides by 1:
a = -11
So, the answer is a = -11.