Excel - create average from indirect

Hello !

On Excel I would like to calculate an average from a different sheet (named "Form") from the entire column B. Does one of you know the exact code ? 

I looked it up on different forums but I was unable to find the right answer, when I found something that made sense, I tried changing the names and values of the code to match my case but it always shows an error...

Thank you in advance!

Amélia