For this problem we know that the inventory for the tirw warehouse is 100 so that represent our starting point for the graph.
And we also know that customers can only purchases tires in complete sets of four. Basically each set of tires we reduce the amount of the inventory on 4. So we can create the following equation:

Where y represent the Tires Remaining and x the sets of tires purchased. Now with this equation we can plot the graph on this way.