Since this is a right triangle, we can use the Pythagorean Theorem to find the missing side length.

$(\operatorname{leg} 1)^2 + (\operatorname{leg} 2)^2 =$ (hypotenuse)^{$^{2}$}
We know one of the leg lengths and the hypotenuse. Substitute the numbers into the equation.

$x^2 + (8)^2 = 17^2$
Now solve for $x$.