To find side $ a $ use formula:
$$ h = \dfrac{ \sqrt{ 3 } \cdot a }{ 2 } $$After substituting $ h = 10 $ we have:
$$ 10 = \dfrac{ \sqrt{ 3 } \cdot a }{ 2 } $$ $$ \sqrt{ 3 } \cdot a = 10 \cdot 2 $$ $$ \sqrt{ 3 } \cdot a = 20 $$ $$ a = \dfrac{ 20 }{ \sqrt{ 3 } } $$ $$ a = \frac{ 20 \sqrt{ 3}}{ 3 } $$