Answer
$-\frac{x+y}{x-y}$
Work Step by Step
$\frac{(a-3)(x+y)}{(3-a)(x-y)}=\frac{-(3-a)(x+y)}{(3-a)(x-y)}=-\frac{3-a}{3-a}\times\frac{x+y}{x-y}=-1\times\frac{x+y}{x-y}=-\frac{x+y}{x-y}$
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