#### Answer

$x=-1$

#### 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 $(-1, 4)$.
The x-coordinate of the point $(-1, 4)$ is $-1$.
Thus, the equation of the line is $x=-1$.