To find side $ a $ use formula:
$$ V = \dfrac{ \sqrt{ 3 } \cdot a ^{ 2 } \cdot l }{ 2 } $$After substituting $ V = 35 $ and $ l = 7 $ we have:
$$ 35 = \dfrac{ \sqrt{ 3 } \cdot a ^{ 2 } \cdot 7 }{ 2 } $$$$ 35 \cdot 2 = \sqrt{ 3 } \cdot a ^{ 2 } \cdot 7 $$$$ 70 = \sqrt{ 3 } \cdot a ^{ 2 } \cdot 7 $$$$ 70 = 7 \sqrt{ 3 } \cdot a ^{ 2 } $$$$ a ^{ 2 } = \dfrac{ 70}{ 7 \sqrt{ 3 } } $$$$ a ^{ 2 } = \frac{ 10 \sqrt{ 3}}{ 3 } $$$$ a = \sqrt{ \frac{ 10 \sqrt{ 3}}{ 3 } } $$$$ a = 2.4028 $$