An athlete runs on a straight track. She starts from rest and runs with a constant acceleration for the first 2 seconds, reaching a speed of 9 ~ms ⁻¹ . She then continues at this constant speed for some time before slowing down to a halt at a constant deceleration. The total time taken, from start to finish, is 12 seconds. If the magnitude of her acceleration is twice the magnitude of deceleration, then what is the t