Review Conceptual Example 6 as background for this problem. A car is traveling to the left, which is the negative direction. The direction of travel remains the same throughout this problem. The car's initial speed is 38.6 m/s, and during a 6.00-second interval, it changes to a final speed of (a)54.2 m/s and (b)34.6 m/s. In each case, find the acceleration (magnitude and algebraic sign)