First, we need to find the least common denominator, the lowest multiple of each number that they would share. Looking at the answer choices, they kinda give you a hint as to what it is. It's 20. This is because each number already in the denominators can be multiplied by another to equal 20.
For 6/4, we multiply both top and bottom by 5. This gives you 30/20.
For -9/5, we multiply both top and bottom by 4. This gives you -36/20.
For -1/2, we multiply both top and bottom by 10. This gives you -10/20.
Your new equation is:
30/20 - 36/20 - 10/20
OR (30 - 36 - 10) / 20
30 - 36 = -6
-6 - 10 = -16
-16/20 can be simplified by dividing both the top and bottom numbers by a greatest common factor of 4.
-4/5 is your answer, option B