Formula help problem with nesting

[Janie]

Hi,
I am having difficulty with the attached formula. I think the problem is in how I am nesting the formula.
I am trying to sum a group of numbers in column A, then minus a number from Column B & column C.
The result of that formula should then be divided by 10*3.

I can get the first portion of the formula correct using @sum(a5:A8)-sum(b8-c8) but can't get the rest to the formula to work.
Appreciate the help.
Janie