To find area $ A $ use formula:
$$ A = \dfrac{ \sqrt{ 3 } \cdot a^2 }{ 4 } $$After substituting $a = 12\, \text{cm}$ we have:
$$ A = \dfrac{ \sqrt{ 3 } \cdot \left( 12\, \text{cm} \right)^{2} }{ 4 } $$ $$ A = \dfrac{ \sqrt{ 3 } \cdot 144\, \text{cm}^2 }{ 4 }$$ $$ A = \dfrac{ 144 \sqrt{ 3 }\, \text{cm}^2 }{ 4 }$$ $$ A = 36 \sqrt{ 3 }\, \text{cm}^2 $$