Eyeballing it, it looks like the center is (2,4), two to the right, four up.
Let's say the minimum point on the circle is (2,1). So a radius is the distance from the center to there. It's a vertical line, 3 units in length.
We plug into the equation
[tex](x-2)^2 + (y - 4)^2 = 3^2[/tex]
Final answer:
[tex](x-2)^2 + (y - 4)^2 = 9[/tex]