To find height $ h $ use formula:
$$ \sin \left( \alpha \right) = \dfrac{ h }{ a } $$After substituting $\alpha = 60^o$ and $a = 12\, \text{cm}$ we have:
$$ \sin( 60^o ) = \dfrac{ h }{ 12 } $$ $$ \frac{\sqrt{ 3 }}{ 2 } = \dfrac{ h }{ 12 } $$$$ h = \frac{\sqrt{ 3 }}{ 2 } \cdot 12 $$$$ h = 6 \sqrt{ 3 } $$