Answer
$9x^2-6x\sqrt{2}+2$
Work Step by Step
Using $(a-b)^2=a^2-2ab+b^2$ or the square of a binomial, then,
\begin{array}{l}
\left( 3x-\sqrt{2} \right)\left( 3x-\sqrt{2} \right)
\\=
\left( 3x-\sqrt{2} \right)^2
\\=
(3x)^2+2(3x)(-\sqrt{2})+(-\sqrt{2})^2
\\=
9x^2-6x\sqrt{2}+2
.\end{array}