#### Answer

$x+3=0$

#### Work Step by Step

RECALL:
A vertical line has an equation of the form $x=h$, where $h$ is the x-intercept. Every point on this line has an x-coordinate of $x=h$.
The given line is vertical, so its equation is in the form $x=h$.
The line passes through the point $(-3, 4)$. Since the x-coordinate of this point is $-3$, every point on the line has $x=-3$.
Thus, the equation of the line is $x = -3$.
This equation can be written as $x+3=0$.