So, this is a right triangle, meaning that if we find 2 sides we can get the third through Pythagorean theorem. We know the radius (I think? English isnt my first language — basically the distance from te center to the edge of the circle) is 8. We have side OQ, which consists of two parts: the radius and a line 9(cm?) long. Seeing as the radius is 8, we can say that side OQ is equal 9 + 8, or 17. We have side OP (8) so we just have to use Pythagorean theorem:
a^2 + b^2 = c^2
c^2 - b^2 = a^2
17^2 - 8^2 = a^2
289 - 64 = a^2
a^2 = 225
a= sqrt(225)
a = 15
