x²+2x-y²+6y-8
=(x²+2x+1)-(y²-6y+9)
=(x+1)²-(y-3)²=0
(x+1+y-3)(x+1-y+3)=0
(x+y-2)(x-y+4)=0
x+y≠2,x+y-2≠0
x-y+4=0
x-y=-4