You think more information is needed
What additional information would you need?
Usually it would matter what portion of the walk was on level ground and what portion was on the slope. Why does it not matter in this case?
Find the average rate for the uphill and downhill portions of the walk. Remember, the average rate is over the same distance each way, and the average rate is NOT the arithmetic mean of the two rates. That is, the rate is NOT 4.5 mph.