Yes it's linear because the rate of change between x and y remain the same. Every increase of 5 in x results in a decrease of 4 in y, as example from x0 to x5, y goes from 18 to 14, from x5 to 10, y goes from 14 to 10. They decide to double the jump and go from x10 to x20, but as proof of it being linear, y also double the amount it decreases going from 4 to 8.