Since percentages represent a part out of one hundred, we can start off by converting 42% to 42/100. Now that we have this, simplify the fraction so that it is in simplest form. This would be 21/50, which is your answer :)
14000+7x=35x (x being the number of books) solve for x and you have your answer. 14000=35x-7x=28x therefore x=14000/28=500 so he needs to sell 500 books before he breaks even.