To find leg $ a $ use formula:
$$ \cos \left( \beta \right) = \dfrac{ a }{ c } $$After substituting $ \beta = 30^o $ and $ c = 2 \sqrt{ 41 } $ we have:
$$ \cos( 30^o ) = \dfrac{ a }{ 2 \sqrt{ 41 } } $$ $$ \frac{\sqrt{ 3 }}{ 2 } = \dfrac{ a }{ 2 \sqrt{ 41 } } $$$$ a = \frac{\sqrt{ 3 }}{ 2 } \cdot 2 \sqrt{ 41 } $$$$ a = \sqrt{ 123 } $$