ANSWER
Only the equation is a function
EXPLANATION
[tex]f(x)=x^2+8[/tex] is graphed above with its inverse [tex]f^{-1}(x)=\pm \sqrt{x-8}[/tex].
We perform the vertical line test for both the equation and its inverse.
The equation [tex]f(x)=x^2+8[/tex] passed the vertical line hence it is a function.
However, [tex]f^{-1}(x)=\pm \sqrt{x-8}[/tex] failed the vertical line test, hence it is not a function.