Answer
$\dfrac{64x^3}{27}$
Work Step by Step
Using $\left(\dfrac{a}{b}\right)^x=\dfrac{a^x}{b^x}$, then the expression, $
\left(\dfrac{4x}{3}\right)^3
$, simplifies to
\begin{array}{l}\require{cancel}
\dfrac{(4x)^3}{3^3}
\\\\=
\dfrac{(4x)^3}{27}
.\end{array}
Using $(ab)^x=a^xb^x$, then the expression, $
\dfrac{(4x)^3}{27}
$, simplifies to
\begin{array}{l}\require{cancel}
\dfrac{4^3x^3}{27}
\\\\=
\dfrac{64x^3}{27}
.\end{array}