Answer
$3,628,800$
Work Step by Step
Using $P(n,r)=\dfrac{n!}{(n-r)!},$ the given expression, $P(
10,9
),$ evaluates to
\begin{array}{l}\require{cancel}
=\dfrac{10!}{(10-9)!}
\\\\=
\dfrac{10!}{1!}
\\\\=
\dfrac{10(9)(8)(7)(6)(5)(4)(3)(2)(1)}{1}
\\\\=
3,628,800
\end{array}