Answer
$72$
Work Step by Step
$P(n,r)$ denotes the number of permutations of a set of $n$ objects taken $r$ at a time and can be found as:
$$P(n,r)=\dfrac{n!}{(n-r)!}$$
Thus, using the formula above with $n=9$ and $r=2$ yields:
$$P(9,2)=\dfrac{9!}{(9-2)!}=\dfrac{9\times 8 \times 7 !}{7!}=72$$
