To find hypotenuse $ c $ use formula:
$$ \cos \left( \beta \right) = \dfrac{ a }{ c } $$After substituting $ \beta = 30^o $ and $ a = 12 $ we have:
$$ \cos( 30^o ) = \dfrac{ 12 }{ c } $$ $$ \frac{\sqrt{ 3 }}{ 2 } = \dfrac{ 12 }{ c } $$ $$ c = \dfrac{ 12 }{ \frac{\sqrt{ 3 }}{ 2 } } $$ $$ c = 8 \sqrt{ 3 } $$