To simplify ratios, we have to divide each part of the ratio by a common factor. The common factor for the first ratio is 3, and the common factor for the second ratio is 9.
[tex]\begin{gathered} 12\colon21\Rightarrow\frac{12}{3}\colon\frac{21}{3}\Rightarrow4\colon7 \\ 9\colon18\Rightarrow\frac{9}{9}\colon\frac{18}{9}\Rightarrow1\colon2 \end{gathered}[/tex]