Answer
$(f^{-1}~o~f^{-1})(4)=472$
Work Step by Step
$f(x)=\frac{1}{8}x-3$
$y=\frac{1}{8}x-3~~$ (Interchange $x$ and $y$):
$x=\frac{1}{8}y-3$
$x+3=\frac{1}{8}y$
$y=8x+24$
$f^{-1}(x)=8x+24$
$(f^{-1}~o~f^{-1})(4)=f^{-1}(f^{-1}(4))=f^{-1}(8(4)+24)=f^{-1}(56)=8(56)+24=472$
