To find angle $ \alpha $ use formula:
$$ \tan \left( \alpha \right) = \dfrac{ a }{ b } $$After substituting $ a = 4 $ and $ b = 9 $ we have:
$$ \tan \left( \alpha \right) = \dfrac{ 4 }{ 9 } $$ $$ \tan \left( \alpha \right) = \frac{ 4 }{ 9 } $$ $$ \alpha = \arctan\left( \frac{ 4 }{ 9 } \right) $$ $$ \alpha = 23.9625^o $$