To find area $ A $ use formula:
$$ A = \dfrac{ 3 \sqrt{ 3 } \cdot a^2 }{ 2 } $$After substituting $a = 7.5\, \text{cm}$ we have:
$$ A = \dfrac{ 3 \sqrt{ 3 } \cdot \left( 7.5\, \text{cm} \right)^{2} }{ 2 } $$ $$ A = \dfrac{ 3 \sqrt{ 3 } \cdot 56.25\, \text{cm}^2 }{ 2 }$$ $$ A = \dfrac{ 292.2836\, \text{cm}^2 }{ 2 }$$ $$ A = 146.1418\, \text{cm}^2 $$