6. $$ \frac{y^2-x^2}{x^2-2xy+y^2} = \frac{(y-x)(y+x)}{(x-y)^2} = \frac{-(x-y)(y+x)}{(x-y)^2} = \frac{-(x+y)}{x-y} = \frac{-x-y}{x-y} $$
Ответ: $$\frac{-x-y}{x-y}$$