Answer
$_9P_2=72$
Work Step by Step
$_nP_r=\frac{n!}{(n-r)!}$
$n=9$ and $r=2$
$_9P_2=\frac{9!}{(9-2)!}=\frac{9!}{7!}$.
But, $9!=9\times8\times(7\times6\times\times4\times3\times2\times1)=9\times8\times7!$
$_9P_2=\frac{9\times8\times7!}{7!}=9\times8=72$
