Part (a) is referencing that you've already seen this problem in the Inclusion-Exclusion chapter. Looking at Example 4.9 and Practice Problem 4.29 we get the general equation that is listed in 7.29. This is a good time to review that the equation given makes sense (using summation notation here) and that you can explain it using PIE.

For (b), the recursive formula is talked about in 7.9. As part of this problem we just want to verify numerically that both equations give the same answer for 8 people.