Answer
$0$
Work Step by Step
RECALL:
$\text{slope} = \frac{\text{change in y}}{\text{change in x}}$
The points on a horizontal line have the same y-value.
This means that for any two given points, the change in y-value is 0.
Since the numerator of $\frac{\text{change in y}}{\text{change in x}}$, for whatever value of the denominator (except 0), the slope will always be equal to 0.
Thus, the slope of a horizontal line is 0.
