Given that

f(x)= $\displaystyle \frac{5-x}{1-x}$

(a) Explain why f has an inverse and show that $\displaystyle f^{-1}=f$.

(b) Evaluate $\displaystyle f^{51}(4)$.

i can do part a,

but have no idea how to start for part b.

any help is appreciated. =)