[tex]\it \mathcal{A}_{\triangle} = \dfrac{\ell^2\sqrt3}{4} =\dfrac{(6\sqrt3)^2\sqrt3}{4}=\dfrac{36\cdot3\sqrt3}{4} =9\cdot3\sqrt3=27\sqrt3\ cm^2[/tex]
[tex]\it\ Fie\ AD\perp BC[/tex]
[tex]\it \mathcal{A} = \dfrac{BC\cdot AD}{2} \Rightarrow 27\sqrt3 = \dfrac{6\sqrt3\cdot AD}{2} \Rightarrow AD = 9 \ cm .[/tex]