Hint: It didn't matter how many male or female borrowers there were.
Answer: 4200 books.
It doesn't matter how many female borrowers there were as 1/2 of the females accepted the books, and those that did accepted 6 books, which is 3 books per female on average.
Similarly it doesn't matter how many male borrowers there were as 3/4 of the males accepted 4 books each, which is again 3 books each.
So both the males and females accepted, on average, 3 books each. And 1400 * 3 = 4200. QED.