[tex]\displaystyle\bf\\cos\,C=\frac{Cateta~alaturata}{Ipotenuza}\\\\cos\,C=\frac{AC}{BC}\\\\AC=BC\times cos\,C\\\\AC=BC\times cos30^o\\\\AC=12\times\frac{\sqrt{3}}{2}\\\\ \boxed{\bf~AC=6\sqrt{3}~cm}[/tex]
