What's next step to simplify this?

The fraction:

$$\frac{y^2-x^2}{x-y}$$

$$-(x+y)$$

but the best I could do was expand it to:

$$\frac{(y+x)(y-x)}{x-y}$$

What's next step?

-

For $x \neq y$, $$\frac{(y+x)(y-x)}{x-y} = \frac{(y+x)(y-x)}{-(y-x)}$$
$$= -(y+x)$$ $$= -(x+y)$$
only if $x \neq y$ –  I. J. Kennedy Feb 4 '11 at 0:55
I.J. Kennedy: Well, the original expression wouldn't have made sense if $x=y$. –  Fredrik Meyer Feb 6 '11 at 15:02