Answer: she begin with 6.
Explanation:
At the beggining she had A books.
She sold half of that, so now she has A/2 books.
She bought 9, so now she has A/2 + 9 books,
And we know that A/2 + 9 = 11 books
So A/2 = 11 - 9 = 3
A = 2*3 = 6
So she started with 6 books