Answer
$36-2x$
Work Step by Step
Given\begin{equation}
(6+\sqrt{2 x})(6-\sqrt{2 x}).
\end{equation} Multiply the radical expressions and simplify.
\begin{equation}
\begin{aligned}
(6+\sqrt{2 x})(6-\sqrt{2 x})& =6^2 -\left( \sqrt{2 x}\right)^2 \\
& =36-2x.
\end{aligned}
\end{equation} Note that the expressions are conjugates of each other. The rule of multiplying conjugate is given by:$$ (A+B)(A-B) = A^2-B^2$$
