To find side $ a $ use formula:
$$ h = \dfrac{ \sqrt{ 3 } \cdot a }{ 2 } $$After substituting $ h = 55 $ we have:
$$ 55 = \dfrac{ \sqrt{ 3 } \cdot a }{ 2 } $$ $$ \sqrt{ 3 } \cdot a = 55 \cdot 2 $$ $$ \sqrt{ 3 } \cdot a = 110 $$ $$ a = \dfrac{ 110 }{ \sqrt{ 3 } } $$ $$ a = \frac{ 110 \sqrt{ 3}}{ 3 } $$