To solve this, first ask yourself what cost will be constant (aka, what cost will not change depending on how many people are renting?). In this case, 5 is constant, because the campground charges $5 regardless of how many people there are.
Now, let x represent the amount of people who come to the campground.
You can say y = 25x +5 because for each additional person, the campground charges $25 more. (Note that whatever x is, does not change the 5)
So y = 25x +5 represents this situation, where x represents how many people are renting and y represents the amount that is charged overall
If you have any questions lmk :)