To find angle $ \gamma $ use Law of Cosines:
$$ c^2 = a^2 + b^2 - 2 \cdot a \cdot b \cdot \cos( \gamma ) $$After substituting we have:
$$ 25^2 = 52^2 + 28^2 - 2 \cdot 52 \cdot 28 \cdot \cos( \gamma ) $$ $$ 625 = 2704 + 784 - 2912 \cos( \gamma ) $$ $$ 2912 \cos( \gamma ) = 2704 + 784 - 625 $$ $$ 2912 \cos( \gamma ) = 2863 $$ $$ 2912 \cos( \gamma ) = 2863 $$ $$ \cos( \gamma ) = \frac{ 409 }{ 416 } $$ $$ \gamma = \arccos{ \left( \frac{ 409 }{ 416 } \right)} $$ $$ \gamma \approx 10.5257^o $$