ANSWER
4
EXPLANATION
The rate of change of a function is the same as the slope of the function. To find that, we use the formula:
[tex]m\text{ = }\frac{y2\text{ - y1}}{x2\text{ - x1}}[/tex]where (x1, y1) and (x2, y2) are two points that the line passes through.
We have the points (1, 15) and (-2, 3):
[tex]\begin{gathered} m\text{ = }\frac{15\text{ - 3}}{1\text{ -(-2)}}\text{ = }\frac{12}{1\text{ + 2}} \\ m\text{ = }\frac{12}{3} \\ m\text{ = 4} \end{gathered}[/tex]That is the slope,
