To find side $ a $ use formula:
$$ \sin \left( \alpha \right) = \dfrac{ h }{ a } $$After substituting $\alpha = 50^o$ and $h = 3\, \text{cm}$ we have:
$$ \sin( 50^o ) = \dfrac{ 3\, \text{cm} }{ a } $$ $$ 0.766 = \dfrac{ 3\, \text{cm} }{ a } $$ $$ a = \dfrac{ 3\, \text{cm} }{ 0.766 } $$ $$ a = 3.9162\, \text{cm} $$