=x^2-2xy+k+y^2

k-2xy=xy

k=3xy

2, discriminant =0

k^2-4*64=0

k^2=256

K= 16 or k=- 16.

3，x？ -2x+y？ +6y+ 10=0

(x- 1)^2+(y+3)^2=0

x- 1=0

x= 1

y+3=0

y=-3