Answer
$\sqrt{\dfrac{26ab}{2b}}$
Work Step by Step
Rationalizing the denominator of $
\sqrt{\dfrac{13a}{2b}}
$ results to
\begin{array}{l}
\sqrt{\dfrac{13a}{2b}}
\cdot
\sqrt{\dfrac{2b}{2b}}
\\\\=
\sqrt{\dfrac{26ab}{4b^2}}
\\\\=
\sqrt{\dfrac{26ab}{2b}}
.\end{array}