Sally is driving along a straight highway in her classic 1965 Mustang. At time t = 0, she is moving at 10 m/s in the positive x-direction, when she passes a signpost that says "x = 50 m". Her acceleration as a function of time is
a = (2.0-.10)i
where a is measured in m/s^2, and t in seconds. At what time does she reverse directions?