Answer
$x^{2}-81$
Work Step by Step
From Page 301, we know that the product of a sum and difference of two terms $x$ and $y$ is given by the following equation: $(x+y)(x-y)=x^{2}-y^{2}$.
Therefore, $(x+9)(x-9)=x^{2}-9^{2}=x^{2}-81$.
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