To find angle $ \alpha $ use formula:
$$ \sin \left( \alpha \right) = \dfrac{ h }{ a } $$After substituting $h = 13.375\, \text{cm}$ and $a = 16\, \text{cm}$ we have:
$$ \sin \left( \alpha \right) = \dfrac{ 13.375\, \text{cm} }{ 16\, \text{cm} } $$ $$ \sin \left( \alpha \right) = 0.8359 $$ $$ \alpha = \arcsin\left( 0.8359 \right) $$ $$ \alpha = 56.7136^o $$