To find side $ a $ use formula:
$$ d_1^2 + d_2^2 = 4 \cdot a^2 $$After substituting $ d_1 = 5 $ and $ d_2 = 5 $ we have:
$$ 5^2 + 5^2 = 4 \cdot a^2 $$ $$ 25 + 25 = 4 \cdot a^2 $$ $$ 4 \cdot a^2 = 50 $$ $$ a^2 = \frac{ 50 }{ 4 } $$ $$ a^2 = \frac{ 25 }{ 2 } $$ $$ a = \sqrt{ \frac{ 25 }{ 2 } } $$$$ a = \frac{ 5 \sqrt{ 2}}{ 2 } $$