Hey there!
A function is like a machine. You put numbers in it and it gives you one number. You can end up with the same output, but you cannot put in the same number multiple times and get different results. It gives you the same number if you do the same one. Functions cannot have multiple outputs for the same input.
We see here that this relation is not a function. So, we can repeat 2, 8, or 3 to get the corresponding y-value and also 0. For example, a could be 2 and it has 2 outputs which would be -5 and 0. This makes it not a function, so our possible values will be 2, 8, and 3.
Have a wonderful day and keep on learning! :D