You have to move the 4 to the other side and change the addition sign to subtraction. After that, you have to subtract 8 and 4 which will give you 4 and 4 divided by 2 is 2 which means that k equals 2.
So each book is an average of 2.8 cm so 120 divided by each book=number of books 120/2.8=42.85 round down since you can't fit .85 of a book on a shelf the answer is 42 books