To find area $ A $ use formula:
$$ A = \dfrac{ \sqrt{ 3 } \cdot a^2 }{ 4 } $$After substituting $ a = 20 \sqrt{ 3 } $ we have:
$$ A = \dfrac{ \sqrt{ 3 } \cdot \left(20 \sqrt{ 3 }\right)^2 }{ 4 } $$ $$ A = \dfrac{ \sqrt{ 3 } \cdot 1200 }{ 4 }$$ $$ A = \dfrac{ 1200 \sqrt{ 3 } }{ 4 }$$ $$ A = 300 \sqrt{ 3 } $$