I think the answer could be A but I'm not so sure because at first Kayla has 6 boxes with x amount of books in the boxes with four left over so the equation would be 6x+4= number of books. So if she then replaces the now five boxes with x amount of books in them and adds four books to each of the five boxes you get 5x+4=number of books so it all equals out to the same number of books on both sides of the equal sign for 6x+4=5x+4