To find side $ a $ use formula:
$$ d_1^2 + d_2^2 = 4 \cdot a^2 $$After substituting $ d_1 = 6 $ and $ d_2 = 3 $ we have:
$$ 6^2 + 3^2 = 4 \cdot a^2 $$ $$ 36 + 9 = 4 \cdot a^2 $$ $$ 4 \cdot a^2 = 45 $$ $$ a^2 = \frac{ 45 }{ 4 } $$ $$ a^2 = \frac{ 45 }{ 4 } $$ $$ a = \sqrt{ \frac{ 45 }{ 4 } } $$$$ a = \frac{ 3 \sqrt{ 5}}{ 2 } $$