A toy train is pushed forward and released at x_0=2.0;{rm m} with a speed of 2.0 {rm m/s}. It rolls at a steady speed for 2.0 {rm s}, then one wheel begins to stick. The train comes to a stop 6.0 {rm m} from the point at which it was released. What is the magnitude of the train's acceleration after its wheel begins to stick?