I think it's C) but the answer should be 6561.
3 groups and 8 people...

Will give you your answer
The perimeter of an object is equal to its sides added up. Because this is a triangle, it will have 3 equal sides. Since the triangle has 3 equal sides, its perimeter will be equal to one of its side lengths * 3.
Because the perimeter will be equal to one of its side lengths * 3, x is equal to one of the triangle's side lengths.
When working with logarithms, remember that a subtraction sign means you can divide.
log(400) - log(4) can be turned into log(400/4), which is further simplified to log(100).
Assuming that he read at the same speed last summer, 2,724 pages.
There you go. I assumed you know how to derrivate!