How do I set a value of a field based on the contents of two other fields, using the Expression Buil

I have added three new fields in the Issue object: the pick lists Category and Request and the date field Due Date.

I have to accomplish the following: depending of a combination of values in Category and Request, the Due Date must be filled with Today + n.

1. Can I do that in the default value section of the field definition or do I need to create workflows for this?

2. What coding do I put in the Expression Builder? I have tried several things but each effort comes back with an error. I simply don't have enough examples to do some copy/pasting.