To find side $ a $ use formula:
$$ d_1^2 + d_2^2 = 4 \cdot a^2 $$After substituting $ d_1 = 4 $ and $ d_2 = 2 $ we have:
$$ 4^2 + 2^2 = 4 \cdot a^2 $$ $$ 16 + 4 = 4 \cdot a^2 $$ $$ 4 \cdot a^2 = 20 $$ $$ a^2 = \frac{ 20 }{ 4 } $$ $$ a^2 = 5 $$ $$ a = \sqrt{ 5 } $$