Answer
$y$ is not a function of $x$
Work Step by Step
Solve the equation for $y$:
$x^2+y^2=4$
$y^2=4-x^2$
$y=±\sqrt {4-x^2}$
So, for each value of $x$ in the domain, except $x=±2$, we will find 2 different values of $y$. For example, if $x=0$:
$y=±\sqrt {4-0^2}=±\sqrt 4=±2$
