standard_description: Interpret the equation y = mx + b as defining a linear function, whose graph is a straight line; give examples of functions that are not linear. For example, the function A = s2 giving the area of a square as a function of its side length is not linear because its graph contains the points (1,1), (2,4) and (3,9), which are not on a straight line.
problem_activity: The estimated size for a population of rabbits and a population of coyotes in a desert habitat are shown in the table. The estimated population sizes were recorded as a part of a long-term study related to the effect of commercial development on native animal species.
|  Years since initial count (       *        n       *       )  |   0    |   3    |   6   |   9    |   12   |   15   |  18   |   21   |   24   |
|----------------------------------------------------------------|--------|--------|-------|--------|--------|--------|-------|--------|--------|
| Estimated number of rabbits (       *        r       *       ) | 14,989 | 10,055 | 5,002 | 10,033 | 15,002 | 10,204 | 4,999 | 10,002 | 14,985 |
| Estimated number of coyotes (       *        c       *       ) | 1,995  | 2,201  | 2,003 | 1,795  | 1,999  | 2,208  | 2,010 | 1,804  | 2,001  |
Below are sketches of each graph, with a curve fitted to the data.
[IMG]
[IMG]
Write the function for each situation and then calculate the two different numbers of years where the population estimate is about 13,000 for rabbits and 2,100 for coyotes.


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