Mayan,
I want to think of the equation in the form
(x  h)^{2} + (y  k)^{2} = r^{2}
This is the circle with centre (h,k) and radius r.
Since (h,k) is on the line I know that
2h  k + 4 = 0 or k = 2h + 4
Also I know that (0,4) and (3,7) are on the circle so the distance from (h,k) to (0,4) is the same as the distance from (h,k) to (3,7). Use the distance formula to write the two distances, set them equal, substitute for k using the equation above and solve for h.
Penny
