First of all, a positive average rate of change means that the function is increasing, so the answer can only be A->B or B->C.
The average rate of change is defined as
i.e. as the ratio between the vertical and horizontal gain.
In order to go from point A to point B, you gain 1 unit horizontally and 5 units vertically. So, the average rate of change is 5/1=5.
In order to go from point B to point C, you gain 1 unit horizontally and 3 units vertically. So, the average rate of change is 3/1=3.
To find the equation of the line, we need to figure out the slope and y-intercept.
To find the y-intercept, look at the number where the line intersects the y-axis. The number is 5. The equation so far is y = ?x + 5.
We still didn't figure out the slope yet. To figure out the slope, pick two points. Use the rise over run method. It is in the image below. The line is going up to the left, so the slope will be negative. Now we know the slope is -1/3. The equation now that we figured out the slope is y = -1/3x + 5.
I hope this helps you! Let me know if I got something wrong.
