Answer
$\text{$y$ is NOT a function of $x$.}$
Work Step by Step
For an equation to be a function: any input $x$ will yield only one output $y$
We start by solving the equation for $y$
Rearranging the equation:
$$y^2=1-x$$
Taking the square root of both sides:
$$y = \pm \sqrt{1-x}$$
If $x=0$
$$y = \pm \sqrt{1-0} = \pm \sqrt{1} = \pm 1$$
As there are two different outputs resulting from the same input, the equation isn't a function
