Question

Is this table a function?

Answer 1

No
You can’t have two different numbers on the same x point. 81 is not able to be a negative 9 and a positive in the same graph

Answer 2

Answer: No, it is not a function

We have x = 81 repeat itself. Any time x repeats, it means we don't have a function.

Put another way, the input x = 81 leads to more than one y output. A function is only possible if any given input leads to exactly one output. The input must be in the domain.