Answer:
 4km, 2km South
Explanation:
His total distance is 3 + 1 which is 4km 
Distance has no direction, it has only magnitude so you add the numbers. 
His displacement is 3km North - 1km South 
2km South
Displacement is a vector quantity with direction. His walk to North and back to South are on the same line. So displacement is the remaining distance left to his starting point.