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[tex]\displaystyle\\\sum_{n=3}^5(-2x-4n)=-2x-4\cdot3+(-2x)-4\cdot4+(-2x)-4\cdot5\\\sum_{n=3}^5(-2x-4n)=-6x-12-16-20\\\sum_{n=3}^5(-2x-4n)=-6x-48[/tex]