A police officer in a stationary car measures your speed by means of an ultrasonic device that emits sound at 39.6 kHz. You approach the officer at 35.0 m/s and the wave reflected by your car interferes with the emitted sound producing beats. What is the frequency of the beats when the speed of sound in air is 343 m/s?

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.