Answer
$x=2$
Work Step by Step
RECALL:
A vertical line has an undefined slope and has an equation of the form $x=h$ where $h$ the x-coordinate of all the points on the line.
The line has an undefined slope therefore the line must be vertical.
The vertical line passes through the point $(2, -1)$.
The x-coordinate of the point $(2, -1)$ is $2$.
Thus, the equation of the line is $x=2$.