A block of mass 2 kg moving on a horizontal surface with speed of 4 ms^$$-$$1 enters a rough surface ranging from x = 0.5 m to x = 1.5 m. The retarding force in this range of rough surface is related to distance by F = $$-$$kx where k = 12 Nm^$$-$$1. The speed of the block as it just crosses the rough surface will be :