To find hypotenuse $ c $ use formula:
$$ \cos \left( \alpha \right) = \dfrac{ b }{ c } $$After substituting $ \alpha = 60^o $ and $ b = 15 $ we have:
$$ \cos( 60^o ) = \dfrac{ 15 }{ c } $$ $$ \frac{ 1 }{ 2 } = \dfrac{ 15 }{ c } $$ $$ c = \dfrac{ 15 }{ \frac{ 1 }{ 2 } } $$ $$ c = 30 $$