Say a locomotive at a constant 18 mi/h comes onto a track (from some other track) that has a passenger train att 100 mi/h on it. At the instant the train is 0.42 miles away from the locomotive, it starts to decelerate. What is the resulting constant acceleration that is needed to just avoid a collision? I have 18t + 0.42 = 100t + 0.5a(t^2). However, I can't solve this! I know there's something simple I haven't seen, so please help. Thanks.