1. Draw a sketch (see attachment)

2. F is the midpoint of AB. Then the center of the circle is placed on a line through F perpendicular to AB.

3. The distance of the center of the circle to AB is called x.

4. You get 2 right triangles with the radius as hypotenuse. (Note that CF = 2) Use Pythagorean theorem:

........[1]

........[2]

5. Solve for x:

........ I've got x = 5

6. Thus

7. I'll leave the rest for you.