To find side $ a $ use formula:
$$ d_1^2 + d_2^2 = 4 \cdot a^2 $$After substituting $ d_1 = 8 $ and $ d_2 = 6 $ we have:
$$ 8^2 + 6^2 = 4 \cdot a^2 $$ $$ 64 + 36 = 4 \cdot a^2 $$ $$ 4 \cdot a^2 = 100 $$ $$ a^2 = \frac{ 100 }{ 4 } $$ $$ a^2 = 25 $$ $$ a = \sqrt{ 25 } $$$$ a = 5 $$