To find area $ A $ use formula:
$$ A = \dfrac{ \sqrt{ 3 } \cdot a^2 }{ 4 } $$After substituting $ a = 15 $ we have:
$$ A = \dfrac{ \sqrt{ 3 } \cdot 15^2 }{ 4 } $$ $$ A = \dfrac{ \sqrt{ 3 } \cdot 225 }{ 4 }$$ $$ A = \dfrac{ 225 \sqrt{ 3 } }{ 4 }$$ $$ A = \frac{ 225 \sqrt{ 3}}{ 4 } $$