Answer :

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

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