## Algebra: A Combined Approach (4th Edition)

$-\frac{1}{x+y}$
Step 1: $\frac{y-x}{x^{2}-y^{2}}$ Step 2: $\frac{y-x}{(x+y)(x-y)}$ Step 3: $\frac{-(x-y)}{(x+y)(x-y)}$ Step 4: Cancelling (x-y) as it is common to both numerator and denominator, we get $\frac{-1}{x+y}$. Step 5: No further simplification is possible, so the final answer is $-\frac{1}{x+y}$.