To find side $ a $ use formula:
$$ d_1^2 + d_2^2 = 4 \cdot a^2 $$After substituting $ d_1 = 12 $ and $ d_2 = 16 $ we have:
$$ 12^2 + 16^2 = 4 \cdot a^2 $$ $$ 144 + 256 = 4 \cdot a^2 $$ $$ 4 \cdot a^2 = 400 $$ $$ a^2 = \frac{ 400 }{ 4 } $$ $$ a^2 = 100 $$ $$ a = \sqrt{ 100 } $$$$ a = 10 $$