#### Answer

$-\frac{1}{x+y}$

#### Work Step by Step

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 $-\frac{1}{x+y}$.