I think it'd be true, since each x-value has only one corresponding y-value, so it would be a function, however it is not a one-to-one function since there are two x-values that correspond to y = -5. You can think of a parabola as an example; every x-value has only one corresponding y-value and so it is a function, however each y-value has two corresponding x-values and thus it is not one-to-one.