You can put this solution on YOUR website! it is f(3)-f(0)/3-0=
9-(1/e^(1.5))-0-(1/1)divided by 3. NOTE, I AM INTERPRETING THE LAST TERM AS 1/e^(x/2), not as 1/(e^x)/2)
(9-0.2231)-(-1)) divided by 3
8.7769+1 divided by 3
3.26 is average value for the above interpretation