A bullet train is traveling at 23.3 m/s when it approaches a slower train 47 meters ahead traveling in the same direction at 6.67 m/s.

If the faster train begins decelerating at 2.1m/s^2 while the slower train continues at constant speed, how soon will they collide?

I got 3.68 seconds.Using the quadriatic formula.

If the faster train begins decelerating at 2.1m/s^2 while the slower train continues at constant speed, at what relative speed will they collide?

I got 8.5 m/s from subtracting 15.2 (what I got from the quadriatic formula)from 6.67 (slower train).