Chapter 3.17, Problem 17.17E

To determine

To prove: That $\left(\begin{array}{c}n\\ k\end{array}\right)\left(\begin{array}{c}k\\ m\end{array}\right)=\left(\begin{array}{c}n\\ m\end{array}\right)\left(\begin{array}{c}n-m\\ k-m\end{array}\right)$ ; where $n\ge k\ge m\ge 0$ be integers.