OA is the radius.
angle OAB = 90° (tangent ⊥ radius)
(OA)square + (AB)square = (OB)square (By Pythagoras’ Theorem)
(OA)square + (15)square = (17)square
(OA)square = (17)square – (15)square
(OA)square = 64
OA = √64
OA = 8

TRY IT YOURSELF!
1) In the diagram WY is the tangent to the circle. O is the centre of the circle at X. Given that angle OYX= 46°, find angle OYX. Answer: 68°

2) PT is the tangent to the circle at A. The centre of the circle is 0. Given that angle TPO = 80°. Find angle CAP. Answer: 36°.