To find side $ a $ use formula:
$$ B = \dfrac{ \sqrt{ 3 } \cdot a^2 }{ 4 }$$After substituting $ B = 3.4 $ we have:
$$ 3.4 = \dfrac{ \sqrt{ 3 } \cdot a^2 }{ 4 }$$ $$ 3.4 \cdot 4 = \sqrt{ 3 } \cdot a^2 $$ $$ 13.6 = \sqrt{ 3 } \cdot a^2 $$ $$ a^2 = \dfrac{ 13.6 }{ \sqrt{ 3 } } $$ $$ a^2 = 7.852 $$ $$ a = \sqrt{ 7.852 } $$$$ a \approx 2.8021 $$