We draw in a radius (in green), label its length R cm:




That's a right triangle and we only need to find its
green hypotenuse R cm which is a radius of the circle.
We just use the Pythagorean theorem:

R² = 9² + 12²

R² = 81 + 144

R² = 225
      ___
 R = Ö225

 R = 15

So the radius is 15 cm.

Edwin