Determine the average rate of change and instantaneous rate of change?

The depth of water at the end of a pier in vacation village varies with the tides throughout the day and can be modeled by the equation D= 1.5cos[0.575(t-3.5)] + 3.8. Determine the average rate of change as t goes from 4.0 to 6.5. Determine the instantaneous rate of change at t=6.5