Answer
$5^{15/28}$
Work Step by Step
Using the laws of exponents, the given expression, $
\left( 5^{5/4} \right)^{3/7}
,$ simplifies to
\begin{array}{l}\require{cancel}
5^{\left( \frac{5}{4} \right) \left( \frac{3}{7} \right)}
\\\\=
5^{\frac{15}{28}}
\\\\=
5^{15/28}
.\end{array}