# What Is the Slope of a Horizontal Line?

In The Slope of a Line, you learned that the slope, or m, of a line describes how rapidly or slowly change is occurring.

Linear Functions have 4 types of slopes: positive, negative slope, zero slope, and undefined slope.

## Real World Example of Negative Slope

Refer to the graph, Horizontal Line, m = 0.  The x-axis represents time, in hours, and the y-axis represents distance, in miles, from Downtown Houston, Texas.

Hurricane Prince, a Category 5 storm, threatens to flood (among other things) the Bayou City in 24 hours. You have the bright idea—along with 2 million other Houstonians—to leave Houston now. You’re on Interstate 45 North, the road that snakes northward to flee anything blowing in from the Gulf of Mexico.

Notice how time is moving. One hour passes, two hours pass, but you’re still one mile away from downtown. Remember, slope is a rate of change. For every two hours that pass, you move zero miles. Because of this, your slope is 0.

## Calculating Zero Slope

Refer to the PDF, Calculate_Zero_Slope to learn how to use a graph and the slope formula to calculate a zero slope. To download free software to view the PDF, visit https://get.adobe.com/reader/.

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