Find an equation of the tangent plane to the parametric surface $x = -3 r \cos \theta$, $y = 1 r \sin \theta$, $z = r$ at the point $\left( -3 \sqrt{2} , 1 \sqrt{2}, 2 \right)$ when $r = 2$, $\theta = \pi / 4$.

z =

Note: Your answer should be an expression of x and y; e.g. "3x - 4y"