The change in y (y being the height of the plant) over 4 weeks was 6 2/3 in.
The average rate of change of the plant's height is therefore
6 2/3 inches
------------------- = 1 2/3 inches/week on average.
4 weeks
The average rate of change of the plant growth per week was 1 2/3 inches. That would be the decrease per week.
