The possible values for x and y is
x__y
-2 -4
-1_-2
0_ 0
1_ 2
2_ 4
It is a function because no x value maps two different y values.
I will also say it not a function because
x^2+y^2< or equal to 20
2^2+2^2< or equal to 20
4+4
8
And
2^2+1^2
4+1
5
So here 2 for x works with different y values
So graph the table I gave given you and state that it is a function and write the reason