So first you do 9 x 2 = 18. So that means that at the end of those 9 months, Jordan has a total of 18 books.
Well, you want to know how many more books does he need before he has bought 25. & that's an easy subtracting question.
So now you do 25 - 19 = 6.
So we now know that Jordan will need to buy 6 more books before he gets to 25 all together.
I hope you understand ! (: