To find angle $ \beta $ use Law of Cosines:
$$ b^2 = a^2 + c^2 - 2 \cdot a \cdot c \cdot \cos( \beta ) $$After substituting we have:
$$ 7^2 = 5^2 + 3^2 - 2 \cdot 5 \cdot 3 \cdot \cos( \beta ) $$ $$ 49 = 25 + 9 - 30 \cos( \beta ) $$ $$ 30 \cos( \beta ) = 25 + 9 - 49 $$ $$ 30 \cos( \beta ) = -15 $$ $$ 30 \cos( \beta ) = -15 $$ $$ \cos( \beta ) = -\frac{ 1 }{ 2 } $$ $$ \beta = \arccos{ \left( -\frac{ 1 }{ 2 } \right)} $$ $$ \beta = 120^o $$