When you want to subtract fractions, you first want to see if they have the same denominator. In this case, there isn't a common denominator so we need to make it so there is. Since 15 is the larger one and a multiple of 3, we can change -6/3 to -30/15 by multiplying the top and bottom by 5. Now the double negatives cancel each other out so instead of -30/15 - (-7/15), it will be -30/15 + 7/15. Add them together and you get -23/15.
