Arunner traveled 15 kilometers north then backtracked 11 kilometers south before stopping. his resultant displacement was

 Resultant displacement is 4 km north.

Explanation:

    Let east represents positive x- axis and north represent positive y - axis. Horizontal component is i and vertical component is j.

   A runner traveled 15 kilometers north then backtracked 11 kilometers south before stopping,

   So initial displacement, 15 kilometers north = 15 j km

        Second displacement, 11 kilometers south = -11 j km

   Total displacement = 15 j -11 j = 4 j km

   Total displacement is 4 km north.