Answer
$a.\quad $the line $x=y$, in the xy-plane
$b.\quad $A plane containing points $(t,t,z) $
(the x- and y-coordinates are equal; z can be any number).
Work Step by Step
$a.\quad $
$z=0$ is the xy-plane, and these are the points on the line $x=y$, in the xy-plane.
$b.\quad $
When the line from part (a) is raised or lowered at will (when the xy plane is raised or lowered), a plane is formed, perpendicular to the xy-plane, containing the above line. All the points in this plane have coordinates $(t,t,z) $ (the x- and y-coordinates are equal, z can be any number).
