Question 458909
a) Find the distance from one point to the other. That will be the length of the diameter.


Take half of that for the radius.
b) Find the midpoint of the diameter. That will be the center of the circle
c)(x-h)^2+(y-k)^2=r^2 where (h,k) is the center and r is the radius
See 
http://www.algebra.com/algebra/homework/Quadratic-relations-and-conic-sections/Quadratic-relations-and-conic-sections.faq.question.374780.html
to follow his example.