I'm not sure how to factorise, please help!! a(x+y) - (x+y)^2 Help would be greatly appreciated, thank you!

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(x + y) is a term that appears in both products so it is called the common term and it can be factored outside.  Then you have (x + y) (a-(x + y)).  This simplifies to: (x + y)(a-x-y)