Answer
$117$
Work Step by Step
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}
