Definite Integrals - Average Values, Integration Rules?

[starless.aeon]

Homework Statement

Find the average value of the function f(t) = (t-3)^2 on [0,6]

Homework Equations

Average value equation (see below)

The Attempt at a Solution

Okay, I have this:

http://img4.imageshack.us/img4/5338/howthecrap.png

But I don't know how it gets to the last step?

 

[Mark44]

You did great up until the end. Substitute the values of the three integrals into what you have in the line above. You should get 1/6*(72 - 6*18 + 9*6). This answer, when simplified, agrees with mine.