a dog runs 300 feet east to fetch a ball and then 300 west to return it to the person who threw it. what is the dog's displacement ? ?

 Distance is a scalar quantity that refers to "how much ground an object has covered" during its motion.Displacement is a vector quantity that refers to "how far out of place an object is"; it is the object's overall change in position.

To calculate displacement, simply draw a vector from your starting point to your final position and solve for the length of this line. If your starting and ending position are the same, like your circular 5K route, then your displacement is 0. In physics, displacement is represented by Δs.
 
 For me to solve this I would need to know the time, but I can give you a handy displacement calculator I used that helped me. 
 
https://www.easycalculation.com/physics/classical-physics/constant-acc-displacement.php

Hope I helped.