Answer
In two-space, $x=4$ is a vertical line parallel to y-axis with x-intercept (4,0).
In three-space, $x=4$ is a plane parallel to y-axis with x-intercept (4,0).
Work Step by Step
In two-space, $x=4$ is a line that extends infinitely in the vertical directions from the x-intercept (4,0).
In three-space, $x=4$ is a plane parallel to the y-axis that extends indefinitely from the x-intercept (4,0). We are essentially taking the 2D line and drawing it to account for all points along the z-axis.
