To find angle $ \alpha $ use formula:
$$ \sin \left( \frac{ \alpha }{ 2 } \right) = \dfrac{ d_2 }{ d_1 } $$After substituting $ d_2 = 15 $ and $ d_1 = 24 $ we have:
$$ \sin \left( \frac{ \alpha }{ 2 } \right) = \dfrac{ 15 }{ 24 } $$ $$ \sin \left( \frac{ \alpha }{ 2 } \right) = \frac{ 5 }{ 8 } $$ $$ \frac{ \alpha }{ 2 } = \arcsin\left( \frac{ 5 }{ 8 } \right) $$ $$ \frac{ \alpha }{ 2 } = 38.6822^o $$$$ \alpha = 38.6822^o \cdot 2 $$$$ \alpha = 77.3644^o $$