The slope is best understood by looking at the units of the independent and dependent variable. The units are always the units on the dependent variable per the units on the independent variable. This problem illustrates this fact.

**Problem 1** The percent *p* of high school seniors who ever used marijuana can be related to *x*, the number of year after 2000, by the equation 25*p* + 21*x* = 1215.

a. Use this model to determine the slope of the graph of this function if *x* is the independent variable.

b. What is the rate of change of the percent of high school seniors using marijuana per year?

This last part indicates that each year, the percentage drops by 0.84 percent.