Answer
$\dfrac{x^3}{125}$
Work Step by Step
Using $\left(\dfrac{a}{b}\right)^x=\dfrac{a^x}{b^x}$, then the expression, $
\left(\dfrac{x}{5}\right)^3
$, simplifies to
\begin{array}{l}\require{cancel}
\dfrac{x^3}{5^3}
\\\\=
\dfrac{x^3}{125}
.\end{array}