To find side $ a $ use Pythagorean Theorem:
$$ a^2 + b^2 = c^2 $$After substituting $ b = 8 $ and $ c = 17 $ we have:
$$ a ^ {\,2} + 8^2 = 17^2 $$ $$ a ^ {\,2} = 17^2 - 8^2 $$ $$ a ^ {\,2} = 289 - 64 $$ $$ a ^ {\,2} = 225 $$ $$ a = \sqrt{ 225 } $$$$ a = 15 $$