Last year, the school library had a total of x books. Over the summer, the library acquired another 42 books and now has a total of 2,402 books. Which equation could be used to find x, the number of books the library had last year?
A.2,402 + x = 42
B.x + 42 = 2,402
C.42x = 2,402
D.x - 42 = 2,402