Answer
$36$
Work Step by Step
Using $_nC_r=\dfrac{n!}{r!(n-r)!},$ the given expression, $
_9C_7
$ evaluates to
\begin{array}{l}\require{cancel}
=\dfrac{9!}{7!(9-7)!}
\\\\=
\dfrac{9!}{7!2!}
\\\\=
\dfrac{9(8)(7!)}{7!(2)(1)}
\\\\=
\dfrac{9(\cancel{8}^4)(\cancel{7!})}{\cancel{7!}(\cancel{2})(1)}
\\\\=
\dfrac{36}{1}
\\\\=
36
\end{array}
