Answer
$\sqrt (m+3n)$
Work Step by Step
We are given that $a^{\frac{m}{n}}=\sqrt[n] a^{m}=(\sqrt[n] a)^{m}$, if all indicated roots are real numbers.
Therefore, $(m+3n)^{\frac{1}{2}}=\sqrt[2] (m+3n)^{1}=\sqrt (m+3n)$.
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