To find side $ a $ use formula:
$$ d_1^2 + d_2^2 = 4 \cdot a^2 $$After substituting $ d_1 = 35 $ and $ d_2 = 20 $ we have:
$$ 35^2 + 20^2 = 4 \cdot a^2 $$ $$ 1225 + 400 = 4 \cdot a^2 $$ $$ 4 \cdot a^2 = 1625 $$ $$ a^2 = \frac{ 1625 }{ 4 } $$ $$ a^2 = \frac{ 1625 }{ 4 } $$ $$ a = \sqrt{ \frac{ 1625 }{ 4 } } $$$$ a = \frac{ 5 \sqrt{ 65}}{ 2 } $$