Answer
$f^{-1} (x)=5x+4$
Work Step by Step
We have $f^{-1} (x) =5x+4$
and $f(f^{-1} (x))=f(5x+4)= \dfrac{(5x+4)-4}{5}=\dfrac{5x}{5}=x$
Now, $f(f^{-1} (x))=f(\dfrac{x-4}{5})= 5(\dfrac{x-4}{5})+4=x-4+4=x$
Thus, we have $f^{-1} (x)=5x+4$
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