Question

Is {(5, –4), (3, –4), (–5, 3), (–4, 5)} a function?

Answer 1

No, because for the same value y = -4, there are 2 values of x: 5 and 3

Answer 2

Yes it is a function. The requirement is that there are not two pairs with the same `x` and different `y`.

This set has no two pairs with the same `x`, so (obviously) it has no two pairs with the same `x` and different `y`.