To find leg $ b $ use formula:
$$ \cos \left( \alpha \right) = \dfrac{ b }{ c } $$After substituting $ \alpha = 30^o $ and $ c = 5 $ we have:
$$ \cos( 30^o ) = \dfrac{ b }{ 5 } $$ $$ \frac{\sqrt{ 3 }}{ 2 } = \dfrac{ b }{ 5 } $$$$ b = \frac{\sqrt{ 3 }}{ 2 } \cdot 5 $$$$ b = \frac{ 5 \sqrt{ 3}}{ 2 } $$