We won't have a function if for same value of x in (x,y) we get different values y.
So first step: figure out k so that the first coordinate (x) is the same:
3k-4=4k | solve for k
k = -4
no check the values y for the elements of the relation
x = 3k-4 = -12-4=-16
so at -16 we get (-16,16) and (-16, 32), which mean for k=-4 the relation is not a function.
Let me know if you have any questions.