Question

race car travels on racetrack at 38m/s and slowa at a constant rate of 20m/s over 10s how far does it move during this time

Answer 1

I assume you mean slows TO 20 m/s over ten seconds.

constant acceleration = (final speed - initial speed )/time =(20-38)/10 = -1.8 m/s^2
d = Vo t + .5 a t^2 = 380 - .9 (100) =290 m
or
you could just use the average speed
(38+20)/2 = 29
29*10 = 290 meters