The unit rate in a graph is how much a unit increases with time. The unit here is Distance (mi) and the time here is Time (h). So to find the unit rate, you divide the y-value with its corresponding x-value (given it is a straight line, which it is in this case).
So for example, let's take the unit 80mi, its corresponding x-value is 2, so 80/2 = 40. Let's do the same for 160mi, its corresponding x-value is 4, so 160/4 = 40 and so on. The value will be the same for all x-y values in the graph.
