To find side $ a $ use Thw Law of Sines:
$$ \dfrac{ a } { \sin( \alpha ) } = \dfrac{ c } { \sin( \gamma ) } $$After substituting $\alpha = 90^o$, $c = 12$ and $\gamma = 25^o$ we have:
$$ \dfrac{ a } { \sin( 90^o ) } = \dfrac{ 12 } { \sin( 25^o ) } $$ $$ \dfrac{ a } { 1 } = \dfrac{ 12 } { 0.4226 } $$ $$ a \cdot 0.4226 = 1 \cdot 12 $$ $$ a \cdot 0.4226 = 12 $$ $$ a = \dfrac{ 12 }{ 0.4226 } $$ $$ a \approx 28.3944 $$