On number 27 you would do as followed;
a-6 7
-------- = ----- You would turn a-6 into -6 so it would look like this
5 12
-6a
---------
5
Then you would turn it into a decimal to make it simpler (That's what I do)
So -6a / 5 becomes 1.2 then you would do the same to 7/12
7/12 = .583333333333 but then you would round it to .6 so ....
1.2a = .6
then you would divide .6 divided by 1.2 so... .6 divided by 1.2 = 0.5
0.5 = 1/2 so a = 1/2
Number 31 is basically the same thing
you turn 5k/10 into 0.5k
the you turn -12k/9 into -1.3
So... .5 divided by -1.3 = -2.6
-2.6 as a fraction is 2 6/10 as a mixed number