We simplify first, then substitute x=2y: (x+2y)−(2x+y)=x+2y−2x−y=y−x=y−2y=−y Alternatively, we could substitute first, then simplify: (x+2y)−(2x+y)=(2y+2y)−(2(2y)+y)=4y−5y=−y
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