Answer:
1st: Find the center of the circle that is concentric to the unknown circle.
2nd: Set up a generic equation of a circle that is centered at the same place as the first one. This one shouldn't have a known radius yet.
3rd: Plug in the given points to find the unknown circle's radius and write out the complete equation.
Explanation:
I wrote these steps after I did the work in the attachment, but I boxed in the work more or less according to the steps. Green is step 1, Orange is step 2, and Purple is step 3.
Let me know if any of it doesn't make sense or my handwriting is hard to read. (Try to ignore the scribbles. I reuse paper when helping people out here.)