\[ \cfrac{12}{\sqrt{15} - 3} = \cfrac{12(\sqrt{15} + 3)}{(\sqrt{15} - 3)(\sqrt{15} + 3)} = \cfrac{12(\sqrt{15} + 3)}{(\sqrt{15})^2 - (3)^2} = \cfrac{12(\sqrt{15} + 3)}{15 - 9} = \cfrac{2(\sqrt{15} + 3)}{1} = 2(\sqrt{15} + 3) \] So, the rationalized form is: \[ 2(\sqrt{15} + 3) \]