It is described in the problem that he will be paying 16.99 per stapler, this can be understood as a linear relationship where both the total amount to pay and the amount of stapler will grow at the same rate (the price of the stapler times the staplers) like this:
Since each stapler costs 16.99, the total cost is 16.99*x, with x representing the number of staplers bought. This works due to that he adds 16.99 for every stapler, and multiplying it adds 16.99 each time he buys a new stapler
https://campussuite-storage.s3.amazonaws.com/prod/735181/c9be791c-b8dc-11e6-bf0d-22000bd8490f/1815482/64a2070e-a58b-11e8-be02-0a844dce770e/file/Homework_Helper-Grade_5_Module_5.pdfStep-by-step explanation: sorry for the link but that should help