To find side $ a $ use formula:
$$ h = \dfrac{ \sqrt{ 3 } \cdot a }{ 2 } $$After substituting $ h = 7.8 $ we have:
$$ 7.8 = \dfrac{ \sqrt{ 3 } \cdot a }{ 2 } $$ $$ \sqrt{ 3 } \cdot a = 7.8 \cdot 2 $$ $$ \sqrt{ 3 } \cdot a = 15.6 $$ $$ a = \dfrac{ 15.6 }{ \sqrt{ 3 } } $$ $$ a \approx 9.0067 $$