Answer
$\frac{1}{a^2}$
Work Step by Step
$\frac{3a}{8}\times\frac{16}{6a^3}$ $\longrightarrow$ factor and eliminate common factors
$\frac{/\!\!\!3a}{/\!\!8}\times\frac{/\!\!2\times/\!\!8}{/\!\!2\times/\!\!\!3a\times a^2}$ $\longrightarrow$ multiply
$\frac{1}{a^2}$
