Hello,
For axis x:
[tex]x^4-2x^2-8=0 \\
==\textgreater\ (x^2)^2-2(x^2)-8=0\\
\Delta=4+4*8=36=6^2 \\0
==\textgreater\ x^2= \dfrac{2+6}{2}\ or \ x^2= \dfrac{2-6}{2}(negative\ to \ refute) \\
==\textgreater \ x^2=4\\
==\textgreater \ (x-2)(x+2)=0\\
[/tex]
For axis y:
x=0==>y=-8
