To find side $ a $ use Thw Law of Sines:
$$ \dfrac{ a } { \sin( \alpha ) } = \dfrac{ c } { \sin( \gamma ) } $$After substituting $\alpha = 52^o$, $c = 21$ and $\gamma = 67^o$ we have:
$$ \dfrac{ a } { \sin( 52^o ) } = \dfrac{ 21 } { \sin( 67^o ) } $$ $$ \dfrac{ a } { 0.788 } = \dfrac{ 21 } { 0.9205 } $$ $$ a \cdot 0.9205 = 0.788 \cdot 21 $$ $$ a \cdot 0.9205 = 16.5482 $$ $$ a = \dfrac{ 16.5482 }{ 0.9205 } $$ $$ a \approx 17.9773 $$