In order to say that the side lengths are a Pythagorean triple, the values should follow or agree the equation c^2 = a^2 + b^2 where c is the hypotenuse of the triangle or the longest side.
Checking the values:
c^2 = 8^2 + 15^2 = 289 = 17^2
Therefore, they are a Pythagorean triple.
