Answer
$-(x+y)$
Work Step by Step
$\frac{x^{2}-y^{2}}{y-x}=\frac{(x+y)(x-y)}{y-x}=\frac{(x+y)(x-y)}{-(x-y)}=\frac{x+y}{-1}\times\frac{x-y}{x-y}=-(x+y)\times1=-(x+y)$
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