Date Tracking, please help

I have attached the file I've been working on. What I need is for the response time column to automatically populate the date the case needs to be seen by based on the date it was received and what is selected in the priority drop down box. If it's Priority 1 we have one day, if it's 2 we have two days, if it's 3 we have three days and weekends don't count on that one. I'm trying to get this together for my staff to use so any help would be greatly appreciated.