[tex]\displaystyle \frac{7a}{9bc} \div\frac{21a}{12b} =\frac{\backslash\!\!\!\!7a}{9 \backslash \!\!\!b \ c} \cdot \frac{4\backslash\!\!\!b}{\backslash\!\!\!\!7a} =\boxed{\frac{4}{9c} }[/tex]
