\(\dfrac{21y}{40}\)

\begin{aligned}
& \frac{7 x}{8} \times \frac{3}{x} \div \frac{5}{y} \\
=& \frac{7 \cancel{x} \times 3}{8 \times \cancel{x}} \div \frac{5}{y} \\
=& \frac{21}{8} \div \frac{5}{y} \\
=& \frac{21 \times y}{8 \times 5} \\
=& \frac{21 y}{40}
\end{aligned}