STEP 1: find side $ a $
To find side $ a $ use formula:
$$ d = 2 \cdot a $$After substituting $d = 51\, \text{cm}$ we have:
$$ 51\, \text{cm} = 2 \cdot a $$ $$ a = \dfrac{ 51\, \text{cm} }{ 2 } $$ $$ a = \frac{ 51 }{ 2 }\, \text{cm} $$STEP 2: find area $ A $
To find area $ A $ use formula:
$$ A = \dfrac{ 3 \sqrt{ 3 } \cdot a^2 }{ 2 } $$After substituting $a = \dfrac{ 51 }{ 2 }\, \text{cm}$ we have:
$$ A = \dfrac{ 3 \sqrt{ 3 } \cdot \left( \frac{ 51 }{ 2 }\, \text{cm} \right)^{2} }{ 2 } $$ $$ A = \dfrac{ 3 \sqrt{ 3 } \cdot \frac{ 2601 }{ 4 }\, \text{cm}^2 }{ 2 }$$ $$ A = \dfrac{ \frac{ 7803 \sqrt{ 3}}{ 4 }\, \text{cm}^2 }{ 2 }$$ $$ A = \frac{ 7803 \sqrt{ 3}}{ 8 }\, \text{cm}^2 $$