Having trouble writing code for average rate of change.

Using the piece wise function:

W(t) = 48+3.64t+0.6363t^2+0.00963t^3 when 1 <= t <= 28 W(t) = -1004+65.8t when 28 < t <= 56

I need to create an m-file that will calculate the average rate over time.

I've made an m-file for the piece wise function itself:

function y = W(x) if 1 <= x && x <= 28 y = 48+3.64*x+0.6363*x.^2+0.00963*x.^3; end if x>28 && x<=56 y =-1004+65.8*x; end

but am having trouble making the code for the average rate of change. I need to be able to plug in two points, (1,25) for example and get an out put. What is the code for this?