as you know, to get the inverse of any expression, we start off by switching the variables about and then solving for "y",
[tex]\bf \stackrel{f(x)}{y}=\cfrac{1}{2}x+4\qquad \qquad \stackrel{inverse}{\underline{x}=\cfrac{1}{2}\underline{y}+4}\implies x-4=\cfrac{1}{2}y
\\\\\\
2x-8=\stackrel{f^{-1}(x)}{y}
\\\\\\
f^{-1}(4)=2(4)-8\implies f^{-1}(4)=0[/tex]
