A great way to divide fractions is to use the "leave it, change it, flip it" method. You leave the first fraction as it is, change the division sign to a multiplication sign, and flip the second fraction so you get the reciprocal. Like this:
-5/7 ÷ 6/7
-5/7 x 6/7
-5/7 x 7/6
Then you multiply through. First -5 x 7, -35, and then 7 x 6, 42, to get -35/42. Because both of these are divisible by 7, we can simplify to get -5/6.