## Intermediate Algebra (6th Edition)

$117$
Using $(a+b)(a-b)=a^2-b^2$ or the product of the sum and difference of like terms , the expression $(\sqrt{6}-9i)(\sqrt{6}+9i)$ simplifies to \begin{array}{l} (\sqrt{6})^2-(9i)^2 \\= 36-81i^2 \\= 36-81(-1) \\= 117 .\end{array}