Draw the line OB. It's another radius of the circle, just like OC, so it's 17. Now look at DB. It's 15, because a radius that's perpendicular to a chord bisects the chord. So we have the triangle ODB. It's a right triangle. The hypotenuse is 17, one leg is 15, and we have to find the other leg.
Dr. Pythagoras tells us exactly how. (One leg squared) + (the other leg squared) = (the hypotenuse squared). It's really tough trying to type math here on my phone, so I'll let you go ahead and finish up the last step.