To take the average, you need to use both of the given z values(-8 and 3) and figure out the slope of the line at each point. slope at each point: -16(-8)= 128 and -16(3)= -48. The average of 128 and -48, calculated as (128-48)/2, is 40. The average rate of change of the function is 40. collegekid1988 · 8 years ago.