How do you find the average rate of change in this.

The graph shows the depth of water W in a reservoir over a one-year period as a function of the number of days x since the beginning of the year. What was the average rate of change of W between x = 0 and x = 100? (Assume that the points lie on the grid lines.) ft/day W (ft) 250 225 200 175 150 1254 100 75 50 25 50 100 150 200 250 300 350 400 + x (days)