Answer:
2 books
Explanation:
your utility function is:
F(U) = TG
In order to maximize your utility you must at least buy 1 book of the Three-Body Problem (T) and 1 book of the Galactic Empires (G). The constraint here is that both T and G must be ≥ 1. Since your budget is only $200, the maximum number of G books that you can buy is 1, or you wouldn't have any money left to buy T books.
So you purchase 1 G book and you have $100 left which you can use to purchase 2 T books.