So, the new brand costs 7 dollars a box. x is the number of boxes. If you let x = 1 box, y = 7 * 1 = 7 dollars per box. So the savings brand is 7 dollars a box, and the original is 10 dollars per box. If he buys 5 boxes in each month, he saves 5 times (10-7) dollars each month or 15 dollars per month.
I hope this helps!!
~Olivia (P.S. Could I get a brainliest answer? I'm working towards Virtuoso and I only need one more brainliest answer to get there! Thank you!!! :) )