To find side $ a $ use formula:

$$ B = \dfrac{ \sqrt{ 3 } \cdot a^2 }{ 4 }$$

After substituting $B = 40\, \text{cm}$ we have:

$$ 40\, \text{cm} = \dfrac{ \sqrt{ 3 } \cdot a^2 }{ 4 }$$ $$ 40\, \text{cm} \cdot 4 = \sqrt{ 3 } \cdot a^2 $$ $$ 160\, \text{cm} = \sqrt{ 3 } \cdot a^2 $$ $$ a^2 = \dfrac{ 160\, \text{cm} }{ \sqrt{ 3 } } $$ $$ a^2 = \frac{ 160 \sqrt{ 3}}{ 3 }\, \text{cm} $$ $$ a = \sqrt{ \frac{ 160 \sqrt{ 3}}{ 3 }\, \text{cm} } $$$$ a \approx 9.6112 $$

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