Answer
$ a.\quad$ The line $x=y$, in the xy-plane.
$ b.\quad$ A plane perpendicular to the xy-plane,
intersecting the xy plane in the line $x=y$.
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).
