To find side $ a $ use formula:
$$ d_1^2 + d_2^2 = 4 \cdot a^2 $$After substituting $ d_1 = 3 $ and $ d_2 = 1 $ we have:
$$ 3^2 + 1^2 = 4 \cdot a^2 $$ $$ 9 + 1 = 4 \cdot a^2 $$ $$ 4 \cdot a^2 = 10 $$ $$ a^2 = \frac{ 10 }{ 4 } $$ $$ a^2 = \frac{ 5 }{ 2 } $$ $$ a = \sqrt{ \frac{ 5 }{ 2 } } $$$$ a = \frac{\sqrt{ 10 }}{ 2 } $$