Hi, it's a little late, but I hope it'll help even now:

To a):

All points P(x,y) on a circle with the center will be described by the equation

With 3 points you generate 3 equations by insertig the coordinates of the points:

Expand these equations and you get:

Substract (vertically!):

So you've got a system of linear equations:

which delivers the solution

Insert these solutions into equation A:

To b)

Insert all your result into the equation mentioned above:

That's the equation of the circle.