Answer: -2
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Draw a vertical line through 4 on the x axis. This vertical line crosses the parabola at some point (which we'll call point A). Draw a horizontal line from point A to the y axis and note how it lands on y = 12. Therefore the point (4,12) is on this parabola.
Repeat the same steps as before to find that (8,4) is also on the parabola
We need to find the slope of the line through (4,12) and (8,4)
m = (y2 - y1)/(x2 - x1)
m = (4-12)/(8 - 4)
m = -8/4
m = -2
The slope of this line is -2 meaning that the average rate of change from x = 4 to x = 8 is -2.
The line goes down 2 units each time you move to the right 1 unit.