Answer
$_9C_2=36$
Work Step by Step
RECALL:
$_nC_r = \dfrac{n!}{r!(n-r)!}$
Use this definition to have:
$\require{cancel}
_9C_2=\dfrac{9!}{2!(9-2)!}
\\_9C_2=\dfrac{9!}{2!(7!)}
\\_9C_2 = \dfrac{9(8)(7!)}{2!(7!)}
\\_9C_2 = \dfrac{9(8)\cancel{(7!)}}{2!\cancel{(7!)}}
\\_9C_2=\dfrac{9(8)}{2}
\\_9C_2=\dfrac{72}{2}
\\_9C_2=36
$
