You can put this solution on YOUR website! A relation is not a function if it assigns more than one y value to any x value in its domain.
For x=0, the equation gives us y=2 and y=-2.
That is enough to say that the equation does not represent y as a function of x.
In fact, the equation represents a circle centered at (0,0) with radius 2.
For any x such that , the equation assigns two values of y.
If you just had the graph corresponding to that equation,
you would know that it is not a function because there is a vertical line x=k that crosses the graph at two points.
(There is really an infinite number of such lines, but one is enough to prove it's not a function).
