Answer
equation: $x=0$
Refer to the image below for the graph.
Work Step by Step
RECALL:
A line with an undefined slope is vertical and has an equation of the form $x=h$ where $h$ is the x-intercept.
Plot the point $P(0, 3)$.
The line has an undefined slope, so it is vertical.
Points on a vertical line share the same x-value.
Thus, plot any point with same x-value as $P$, then connect the points using a straight line to complete the graph.
(refer to the image below for the graph).
The equation of the line is $x=0$.