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Work Shown:
x = year
y = amount of hydrocarbon emissions in grams, per mile driven
m = rate of change = slope
m = (y2 - y1)/(x2 - x1)
m = (0.8 - 3.4)/(2007 - 1972)
m = -2.6/35
m = -0.0742857 approximately
The approximate rate of change is about -0.0742857 grams per year. The negative rate of change indicates the per mile hydrocarbon amount has gone down by about 0.0742857 grams each year.