A 3.50 ? 10^5 kg subway train is brought to a stop from a speedof 0.500 m/s in 0

A 3.50 ? 10^5 kg subway train is brought to a stop from a speedof 0.500 m/s in 0.700 m by a large spring bumper at the end of itstrack. What is the force constant /k/ of the spring? 1[INPUT]N/m