Answer
$\dfrac{1}{15}x^{4}$
Work Step by Step
The product of the given expression, $
\left( -\dfrac{1}{5}x^3 \right)\left( -\dfrac{1}{3}x \right)
$, is
\begin{array}{l}\require{cancel}
\left( -\dfrac{1}{5} \right)\left( -\dfrac{1}{3} \right)x^{3+1}
\\\\=
\dfrac{1}{15}x^{4}
.\end{array}