To find side $ a $ use formula:
$$ A = \dfrac{ 3 \sqrt{ 3 } \cdot a^2 }{ 2 }$$After substituting $ A = \frac{ 1299 }{ 5 } $ we have:
$$ \frac{ 1299 }{ 5 } = \dfrac{ 3 \sqrt{ 3 } \cdot a^2 }{ 2 }$$ $$ \frac{ 1299 }{ 5 } \cdot 2 = 3 \sqrt{ 3 } \cdot a^2 $$ $$ \frac{ 2598 }{ 5 } = 3 \sqrt{ 3 } \cdot a^2 $$ $$ a^2 = \dfrac{ \frac{ 2598 }{ 5 } }{ 3 \sqrt{ 3 } } $$ $$ a^2 = \frac{ 866 \sqrt{ 3}}{ 15 } $$ $$ a = \sqrt{ \frac{ 866 \sqrt{ 3}}{ 15 } } $$$$ a \approx 9.9999 $$