Answer :
[tex]\frac{4}{12}=\frac{x}{9}[/tex]
cross multiply
[tex]\begin{gathered} 9\times4=x\times12 \\ 36=12x \\ \text{divide both sides by 12} \\ \frac{36}{12}=\frac{12x}{12} \\ x=3 \end{gathered}[/tex]