The teammates are 15 but 6 of them do not want the photo, so the number of the teammates who wants the photo is 15 - 6.
Now, each photo cost $2 adn the number of photo is equal to the number of teammates who wants the photo which is (15-6), so the expression for the total cost is:
We have to multiply the cost of each photo for the number of photo Greta will buy.