Answer
$a.\quad z=1$
$b.\quad x=3$
$c.\quad y=-1$
Work Step by Step
$ a.\quad$
A plane parallel to the xy-plane is $z=k,\ \ k\in \mathbb{R}.$
The point $(3,-1,1)$ has the z-coordinate equal to $1$, so the plane is $z=1$.
$ b.\quad$
A plane parallel to the yz-plane is $x=k,\ \ k\in \mathbb{R}.$
The point $(3,-1,1)$ has the x-coordinate equal to $3$, so the plane is $x=3$.
$ c.\quad$
A plane parallel to the xz-plane is $y=k,\ \ k\in \mathbb{R}.$
The point $(3,-1,1)$ has the y-coordinate equal to $-1$, so the plane is $y=-1$.
