Hi,
The standard equation of a circle with center C(h,k) and radius r is :
x^2 + y^2 - 6x - 10y + 30 = 0
x^2 -6x + 9 + y^2 -10y +25 + 30 -9 -25 = 0
(x^2 -6x + 9) + (y^2 -10y +25) + (30 -9 -25) = 0
(x-3)^2 + (y-5)^2 -4 = 0
(x-3)^2 + (y-5)^2 = 2^2
sum of the coordinates(3,5) of the center of the circle
3 + 5 = 8