find the equation of the circle passing through (-1,2)(2,6)(6,3)

A general equation could be written:

(x-a)^2 + (y-b)^2 = c^2
where c is the radius of the circle.
You can use the coordinates of the three given points to get three equations in three unknowns.
(-1-a)^2 + (2-b)^2 = c^2
(2-a)^2 + (6-b)^2 = c^2
(6-a)^2 + (3-b)^2 = c^2
Then solve for a, b and c