Kenneth Rosen Edition 7 Exercise 6.4 Question 20 (Page No. 421)

Question

Suppose that $k$ and $n$ are integers with $1 \leq k<n.$ Prove the hexagon identity $\binom{n-1}{k-1}\binom{n}{k+1}\binom{n+1}{k} = \binom{n-1}{k}\binom{n}{k-1}\binom{n+1}{k+1},$ which relates terms in Pascal’s triangle that form a hexagon.