as you already know, to get the inverse of any expression we start off by doing a quick switcheroo on the variables, and then solve for "y".
[tex]\bf y = 4x^2-8\implies \stackrel{\textit{quick switcheroo}}{\underline{x} = 4\underline{y}^2-8}\implies x+8=4y^2\implies \cfrac{x+8}{4}=y^2 \\\\\\ \sqrt{\cfrac{x+8}{4}}=y\implies \cfrac{\sqrt{x+8}}{\sqrt{4}}=y\implies \cfrac{\sqrt{x+8}}{2}=\stackrel{f^{-1}(x)}{y}[/tex]
