x的平方加y的平方减4x加6y加13等于0
x^2 + y^2 -4x +6y +13=0
(x^2 - 4x + 4) +(y^2 +6y +9=0
(x-2)^2 +(y+3)^2 =0
x-2 =0, y+3 =0
x=2, y= -3
x+y =-1