First subtract the weight of the suitcase from the total weight they case can weigh when full:
50 - 5.7 = 44.3 pounds
This means she can put 44. 3 pounds of books in the suitcase.
Now find the number of books:
Divide the total Weight of what she can fit in the suitcase by the weight of each book:
44.3 / 2.4 = 18.458
Because the answer has a decimal you round down to the whole number, because if you rounded up it would weigh more than allowed.
The answer is she can carry 18 books