Answer
$\dfrac{1}{8}a^5$
Work Step by Step
Multiplying the constants and multiplying the variables, the given expression, $
\left( -\dfrac{1}{2}a^3 \right)\left( -\dfrac{1}{4}a^2 \right)
,$ simplifies to
\begin{array}{l}\require{cancel}
\left( -\dfrac{1}{2} \right)\left( -\dfrac{1}{4} \right)a^{3+2}
\\\\=
\dfrac{1}{8}a^5
.\end{array}