The leftmost graph is a 15 mph rate. Since the biker starts at 0 miles traveled after 0 hours, and the y-value at x=1 hour is 15, you can say that the person biked 15 miles for the hour.
The rightmost graph is a 23 mph rate. The 'distance' from 2 hours to 3 hours is 1 hour; additionally, the 'distance' from 69 miles to 46 miles is 23 miles, so you can see that in the 1 hour of travel, 23 miles have been biked.
