To find area $ A $ use formula:
$$ A = \dfrac{ 3 \sqrt{ 3 } \cdot a^2 }{ 2 } $$After substituting $a = 3\, \text{cm}$ we have:
$$ A = \dfrac{ 3 \sqrt{ 3 } \cdot \left( 3\, \text{cm} \right)^{2} }{ 2 } $$ $$ A = \dfrac{ 3 \sqrt{ 3 } \cdot 9\, \text{cm}^2 }{ 2 }$$ $$ A = \dfrac{ 27 \sqrt{ 3 }\, \text{cm}^2 }{ 2 }$$ $$ A = \frac{ 27 \sqrt{ 3}}{ 2 }\, \text{cm}^2 $$