At what distance along the central perpendicular axis of a uniformly charged plastic disk of radius 0.390 m is the magnitude of the electric field equal to one-half the magnitude of the field at the center of the surface of the disk?

Radius, the distance from the centre = 0.390
 Electric field is equal to half of the magnitude. E2 = E / 2
 Given
E1 = E2 E1 = k x Q / r^2
  E2 = (k x Q / r2^2) / 2
  Equating the both we get 2 x r^2 = r2^2
 r2 = square root of (2 x r1^2) = square root of (2) x r = 1.414 x 0.390
  r2 = 1.414 x 0.390 = 0.551 m

You need to be more specific.  i really don't know what a "motion map" is.you can draw a graph of an object's position, distance, displacement, speed,velocity, or acceleration versus time, or you can draw a graph of the object's x-position versus its y-position and not give any information about time at all.if the graph presents position or displacement against time,  then constant velocity is a straight line . . i.e. a line that has  constant slope.if the graph shows speed against time, then constant velocityis a horizontal piece of line.if the graph shows acceleration against time, then constant  velocity is a piece of line that lies on the x-axis . . it's value is zero.