The question is asking you to come up with a real-life example of something that can be represented as 15+ c= 17.50. If I were you I would do sales tax. You bought an item with a 15 dollar price tag, but when you go to check out 2.50 are added in tax to equal 17.50. to solve subtract 15 from 17.50: c= 2.50. Your variable (c) represents the sales tax. Hope that helps! :)