Answer
$\dfrac{3x}{7y}$
Work Step by Step
Cancelling the common factor between the numerator and the denominator, the given expression, $
\dfrac{9x}{21y}
,$ simplifies to
\begin{array}{l}\require{cancel}
\dfrac{\cancel{3}\cdot3x}{\cancel{3}\cdot7y}
\\\\=
\dfrac{3x}{7y}
\end{array}
