A freight car of mass 1800 kg is timed at 4.2m/s just after it runs into a spring-loaded bumper at the end of the track. The bumper consists of an 800 kg mass that the car runs into, and a pair of large springs. The car travels 2.4m before coming to rest.

a) using an appropriate conservation of law, find the speed of the freight car before it struck the bumper.
b) find the time taken for the car to be brought to rest.