Step-by-step explanation: At first glance of the question, it doesn't seem that the order matters, which would make it a question of how many combinations there are. The simple way to look at it is that there are 9 trees and he's planting 8, leaving out 1. Since there are 9 possible trees to leave out, there are 9 possible combinations of which 8 trees to plant.
The formula would be 9!/([9-8]!*8!) = 9
If the question is indeed asking for permutations (order matters), since you are choosing 8 out of 9 the formula would be 9!/(9-8)! = 9!/1! = 9! = 362,880
