We are told that x represents "the number of days after the area has been treated."
We have a function for predicting the number of termites left in an area after x number of days:
f(x) = 10,000 - 1,500x
Let's plug 5 in for x, since we want to know the number of termites left in an area after 5 days:
f(5) = 10,000 - (1,500)(5)
To see what this equals, first we have to do the multiplication: (1,500)(5) = 7500. So:
f(5) = 10,000 - 7500 = 2,500
So, to answer the question, after 5 days, it is predicted that there will be 2,500 termites in the area.
I hope this helps you!