Answer :

(x+y-3)(x+y+3) = x^2+xy+3x+xy+y^2+3y-3x-3y-9 = -9+x^2+y^2+2xy
[tex](x+y-3)(x+y+3)=\\ (x+y)^2-9=\\ x^2+2xy+y^2-9[/tex]

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