[tex]\\ \sf\longmapsto \dfrac{3}{9}=\dfrac{x}{12}[/tex]
[tex]\\ \sf\longmapsto \dfrac{1}{3}=\dfrac{x}{12}[/tex]
[tex]\\ \sf\longmapsto x=\dfrac{12}{3}[/tex]
[tex]\\ \sf\longmapsto x=4[/tex]
