In a function, for every x, there can be only one y. If multiple x values get the same y value, it doesn't necessarily mean it's not a function, but think about it like this:
f(x) = 3x
If I plugged in 2 to that equation, the only number I can get is 6. If I can get any number aside from 6 from f(2), then it doesn't follow the rules of a function.
In the case of your problem, all x values plugged in yield -6, but, a possible function for that would be:
f(x) = x - x - 6
f(x) = -6
Therefore, the given relation is indeed a function.
Hope this helps! :)