[Signals and systems] I'm really lost about the math for calculating the energy of a signal

I know this is the formula:

http://i.imgur.com/f9ndKiw.jpg

But what does the math look like for a function like 3rect(3t)? It's driving me fucking insane because the prof didn't show us any examples for it other than cos(t)... And I haven't found any youtube videos that show an example problem with this formula either. Not even this book has an example for the fucking equation (seriously!).

Please help!