#### Answer

$3b\sqrt[7]{a^5}$

#### Work Step by Step

Using the laws of exponents, the given expression, $
\dfrac{3b}{a^{-5/7}}
,$ is equivalent to
\begin{array}{l}\require{cancel}
3ba^{5/7}
.\end{array}
Using $a^{m/n}=\sqrt[n]{a^m}=(\sqrt[n]{a})^m$, the expression, $
3ba^{5/7},$ simplifies to
\begin{array}{l}\require{cancel}
3b\sqrt[7]{a^5}
.\end{array}