From the information, it seems the runner is slowing down because the speed at 1st hour is 5km/h but at 3rd hour become 3km/h.
If v= velocity, v0= initial speed, t= time, and the a=acceleration then the function would be: v= v0 + a * t
To find the acceleration you need to do this equation: acceleration= velocity1- velocity3 / t3-t1 a = (3km/h-5km/h)/ (3 hour- 1 hour) a = (-2km/h)/2hour= -1 km/hour^2
After that you need to find the initial speed. Try to put the 1st hour variable into the full equation. It would look like this v= v0 + a * t 5km/h= v0 + (-1 km/hour^2 * 1 hour) v0= 5km/h + 1km/h v0= 6km/h
Then the full function would be: v= 6km/h - 1km/h^2 * t
The graph would look like a backslash(\) from 6 gradually go down to 1.
