How to use sigma problem for multiple server excel

Hi,

I'm new to the forum but am looking for help in Excel. My problem for my business course (image attached) is how to find Po (Probability no customers are in the system) given the average arrival rate, average service rate, and number of services. My main problem really is how to use sigma in excel with factorials and right parentheses in the right places. Any help would be appreciated! Thanks.

J