## Elementary Algebra

$\dfrac{3x}{7y}$
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}