x平方加y平方减2xy减6x加6y加9=0
x^2+y^2-2xy-6x+6y+9=0
(x^2+y^2-2xy)-6(x-y)+9=0
(x-y)^2-6(x-y)+9=0
(x-y-3)^2=0
x-y-3=0
x-y=3