To find side $ a $ use formula:
$$ A = \dfrac{ \sqrt{ 3 } \cdot a^2 }{ 4 }$$After substituting $ A = \frac{ 49 \sqrt{ 3}}{ 4 } $ we have:
$$ \frac{ 49 \sqrt{ 3}}{ 4 } = \dfrac{ \sqrt{ 3 } \cdot a^2 }{ 4 }$$ $$ \frac{ 49 \sqrt{ 3}}{ 4 } \cdot 4 = \sqrt{ 3 } \cdot a^2 $$ $$ 49 \sqrt{ 3 } = \sqrt{ 3 } \cdot a^2 $$ $$ a^2 = \dfrac{ 49 \sqrt{ 3 } }{ \sqrt{ 3 } } $$ $$ a^2 = 49 $$ $$ a = \sqrt{ 49 } $$ $$ a = 7 $$