To find area $ A $ use formula:
$$ A = \dfrac{ 3 \sqrt{ 3 } \cdot a^2 }{ 2 } $$After substituting $ a = \frac{ 15 }{ 2 } $ we have:
$$ A = \dfrac{ 3 \sqrt{ 3 } \cdot \left(\frac{ 15 }{ 2 }\right)^2 }{ 2 } $$ $$ A = \dfrac{ 3 \sqrt{ 3 } \cdot \frac{ 225 }{ 4 } }{ 2 }$$ $$ A = \dfrac{ \frac{ 675 \sqrt{ 3}}{ 4 } }{ 2 }$$ $$ A = \frac{ 675 \sqrt{ 3}}{ 8 } $$