Answer
$_9C_4=126$
Work Step by Step
RECALL:
$_nC_r=\dfrac{n!}{r!(n-r)!}$
Use this to have:
$\require{cancel}\\_9C_4=\dfrac{9!}{4!(9-4)!}
\\_9C_4=\dfrac{9!}{4!(5!)}
\\_9C_4=\dfrac{9(8)(7)(6)(5!)}{24(5!)}
\\_9C_4=\dfrac{9(8)(7)(6)\cancel{(5!)}}{24\cancel{(5!)}}
\\_9C_4=\dfrac{9(8)(7)\cancel{(6)}}{\cancel{24}4}
\\_9C_4=\dfrac{9\cancel{(8)}2(7)}{\cancel{4}}
\\_9C_4=126$
