The radius is 8.
Step-by-step explanation:
Because OA is a radius and AB is a tangent, Angle A is a right angle. So you can use Pythagorean theorem (shortcut Pythagorean triple)
OA^2 + 15^2 = 17^2
OA^2 + 225 = 289
OA^2 = 64
OA = 8
OA is a radius, the radius = 8