To find area $ A $ use formula:
$$ A = \dfrac{ 3 \sqrt{ 3 } \cdot a^2 }{ 2 } $$After substituting $ a = \frac{ 155 }{ 4 } $ we have:
$$ A = \dfrac{ 3 \sqrt{ 3 } \cdot \left(\frac{ 155 }{ 4 }\right)^2 }{ 2 } $$ $$ A = \dfrac{ 3 \sqrt{ 3 } \cdot \frac{ 24025 }{ 16 } }{ 2 }$$ $$ A = \dfrac{ \frac{ 72075 \sqrt{ 3}}{ 16 } }{ 2 }$$ $$ A = \frac{ 72075 \sqrt{ 3}}{ 32 } $$