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:
5. Solve for x:
........ I've got x = 5
7. I'll leave the rest for you.