To find the center of the circle, complete the square for both x and y.
x2 + y2 = 4x - 12y - 39 ---> (x2 - 4x + ...) + (y2 + 12y + ...) = -39
(x2 - 4x + 4) + (y2 + 12y + 36) = -39 + 4 + 36
(x - 2)2 + (y+ 6)2 = 1
The center is: (2, -6)
The radius is 1