your answer should be 8
because when you use Pythagorean theorem (a²+b²=c²)
you said side "c" ,which i am assuming is the hypotenuse, was 17 and side "b" is 15
so...
15²+x²= 17²
which is...
225+x²=289
then you solve the equation
you would subtract 225 from 289 which = 64
then you end up with
x²=64
but you need "X" by its self...
so you would √64 to get "X" by its self...
which is 8
I hope this helped!