Answer
$\sqrt[]{x^2-a^2}$
Work Step by Step
Using $\sqrt[a]{x}\sqrt[a]{y}=\sqrt[a]{xy}$, the given expression, $
\sqrt[]{x-a}\sqrt[]{x+a}
,$ simplifies to
\begin{array}{l}\require{cancel}
\sqrt[]{(x-a)(x+a)}
\\\\=
\sqrt[]{x^2-a^2}
.\end{array}