5. The radius of the circle is half the length of the diameter. Do you know a formula for the length of a line segment? If so, plug in the two endpoints, find the length of the diameter and divide by 2.
6. The coordinates of the center of the circle are those of the point at the midpoint of the line segment joining your two points. Do you have a formula for the midpoint of two points? If so, plug those two points in and you will get the midpoint of your circle.