The white parts are blanks that I have to write numbers in.

Answer:
8 sets of clothing
7 days
12 sets of clothing
20 sets of clothing
Step-by-step explanation:
For any vacation, William takes clothing for three extra days, in case he has a delayed trip.
Therefore, for 5 days of vacation, he will take (5 + 3) = 8 sets of clothing.
Again, if he takes 10 sets of clothing then the vacation will be of (10 - 3) = 7 days.
And, for 9 days of vacation, he will take (9 + 3) = 12 sets of clothing.
If William decides to take a vacation that lasts 17 days then he will bring (17 + 3) = 20 sets of clothing. (Answer)