This problem cannot be completed without the accompanying figure. I assume the figure is a right triangle and if so you would use the pythagorean theorem. However, without knowing if 17 is the hypotenuse or if we are finding the hypotenuse, it is impossible to complete the problem. However I will complete the problem both ways and you can choose the problem that is correct for your figure. Lets assume that 17 is the hypotenuse. We know the pythagorean theorem is a^2 + b^2 = c^2 where c is the hypotenuse.
Then we substitute 17 in for c, and 8 in for a.
Simplify by subtracting 8^2 from both sides.
Take the squareroot of both sides, since we know it is a length we only need the positive answer.
The other way to solve the problem if 17 is not the hypotenuse then we put 17 in for a and 8 in for b.
Take the squareroot of each side.
Since this does not come out evenly, I would assume your problem is the first.