Answer
The slope is not defined.
Work Step by Step
m is the slope of the line.
$m=\frac{\triangle y}{\triangle x}=\frac{y_2-y_1}{x_2-x_1}=\frac{d-b}{a-a}=\frac{d-b}{0}$ The denominator is $0$ therefore the slope is not defined. (The line is vertical)
