Look at the distance each car moved and the time it took to move that distance. The direction each car moved does not affect its speed.
Notice that each car moved for 10 hours. The car that moved 140 miles moved the shortest distance in that time. So, that car must have moved at the lowest speed.
Compare the motion of three cars. Which car was moving at the lowest speed?
A: a car that moved 140miles west in 10hours
B: a car that moved 640miles east in 10hours
C: a car that moved 355miles east in 10hours