find the inverse of the function and then determine whether the inverse is a function.
y=x^2+4

as you already know, we start off by doing a quick switcheroo on the variables, and then we solve for y to get the inverse,
[tex] \bf y=x^2+4\implies \stackrel{switcheroo}{x = y^2+4}\implies x-4=y^2\implies \pm\sqrt{x-4}=\stackrel{f^{-1}(x)}{y} [/tex]
and if you run a vertical line test on that expression, you'll find that it doesn't pass it, meaning is not a function.