There is a rule for functions:
One input(x-value) can only have one output(y-value).
If one input has more than one output, it is not a function.
(This doesn't apply to outputs, one output can have more than one input and still be a function)
{(-9,0), (-8,8), (-1,-8), (2,8)}
This is a function because each input has only one output.
{(-6, -2),(-7,-7),(-5,-2), (-9,-8)}
This is a function because each input has only one output.
{(9,6), (-3,-9),(-8, -1), (9,5)}
This is not a function because the input 9 has more than one output. (9, 6), (9, 5)
{(-1,-8), (4,1), (-7,-3), (-3,5)}
This is a function because each input has only one output.