Answer
The required solution is $\left( \begin{align}
& n \\
& r \\
\end{align} \right)=\underline{\frac{n!}{r!\left( n-r \right)!}}$
Work Step by Step
We know that for non-negative integers n and r, with$n\ge r$, the expression $\left( \begin{align}
& n \\
& r \\
\end{align} \right)$ (read as “n above r”) is called the binomial coefficient and is defined by:
$\left( \begin{align}
& n \\
& r \\
\end{align} \right)=\frac{n!}{r!\left( n-r \right)!}$
