To find side $ a $ use formula:
$$ d_1^2 + d_2^2 = 4 \cdot a^2 $$After substituting $ d_1 = 8 $ and $ d_2 = 15 $ we have:
$$ 8^2 + 15^2 = 4 \cdot a^2 $$ $$ 64 + 225 = 4 \cdot a^2 $$ $$ 4 \cdot a^2 = 289 $$ $$ a^2 = \frac{ 289 }{ 4 } $$ $$ a^2 = \frac{ 289 }{ 4 } $$ $$ a = \sqrt{ \frac{ 289 }{ 4 } } $$$$ a = \frac{ 17 }{ 2 } $$