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.