To find side $ a $ use formula:
$$ h = \dfrac{ \sqrt{ 3 } \cdot a }{ 2 } $$After substituting $ h = 2.6 $ we have:
$$ 2.6 = \dfrac{ \sqrt{ 3 } \cdot a }{ 2 } $$ $$ \sqrt{ 3 } \cdot a = 2.6 \cdot 2 $$ $$ \sqrt{ 3 } \cdot a = 5.2 $$ $$ a = \dfrac{ 5.2 }{ \sqrt{ 3 } } $$ $$ a \approx 3.0022 $$