Answer
$z\leq 0$
Work Step by Step
The xy plane is $z=0.$
where x and y can be any real numbers, without restriction.
$z=k$ are planes parallel to the xy plane, and we want the planes below or on the xy-plane.
That is, we want $ z=k$ with $ k\leq 0$
or,
$z\leq 0$
