Answer:
The radius is of length 12.
Explanation:
A line that is tangent to a circle forms a right angle with the radius and the point of tangency. If you look closely at the diagram, you can see that a right triangle has been formed, with leg lengths r and 16 and a hypotenuse of length r + 8. Therefore, we can use the Pythagorean Theorem to find the length of the radius. I have done out the work in the attached file.
Hope this helps!