What use do we have for motion diagrams?

A motion diagram represents the motion of an object by displaying its location at various equally spaced times on the same diagram. Motion diagrams are a pictorial description of an object's motion. They show an object's position and velocity initially and present several spots in the center of the diagram.

Explanation:

a and b sometimes have a positive value

if the person is shown to have +90n, then ans is -10n.

if the person is shown to have -90n, then ans is +10n.