To find side $ a $ use formula:
$$ d_1^2 + d_2^2 = 4 \cdot a^2 $$After substituting $ d_1 = 11 $ and $ d_2 = 8 $ we have:
$$ 11^2 + 8^2 = 4 \cdot a^2 $$ $$ 121 + 64 = 4 \cdot a^2 $$ $$ 4 \cdot a^2 = 185 $$ $$ a^2 = \frac{ 185 }{ 4 } $$ $$ a^2 = \frac{ 185 }{ 4 } $$ $$ a = \sqrt{ \frac{ 185 }{ 4 } } $$$$ a = \frac{\sqrt{ 185 }}{ 2 } $$