cheap answer is, you multiply one fraction by the other's denominator, so let's do that with both of these fellows.
[tex] \bf \qquad \cfrac{11}{\boxed{21}}~\hspace{21em}\cfrac{5}{\boxed{9}}\\\\[-0.35em]
~\dotfill\\\\
\cfrac{11\cdot 9}{21\cdot 9}\implies \cfrac{99}{189}~\hspace{15em}\cfrac{5\cdot 21}{9\cdot 21}\implies \cfrac{105}{189} [/tex]
