The slope of a horizontal line is 0.

Explanation:

• A horizontal line runs left to right and does not go up or down.
• Slope is calculated as the "rise over run," or the change in y divided by the change in x.
• Since there is no change in y (rise = 0), the slope = 0 / (change in x) = 0.

So, horizontal lines always have a slope of 0.