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