To find area $ A $ use formula:
$$ A = \dfrac{ 3 \sqrt{ 3 } \cdot a^2 }{ 2 } $$After substituting $a = 16.6\, \text{cm}$ we have:
$$ A = \dfrac{ 3 \sqrt{ 3 } \cdot \left( 16.6\, \text{cm} \right)^{2} }{ 2 } $$ $$ A = \dfrac{ 3 \sqrt{ 3 } \cdot 275.56\, \text{cm}^2 }{ 2 }$$ $$ A = \dfrac{ 1431.8518\, \text{cm}^2 }{ 2 }$$ $$ A = 715.9259\, \text{cm}^2 $$