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Consider, for concreteness, the system to be a mass on a horizontal spring. The mass then takes $t=.25\;{\textrm s}$ to go from the far right to the far left, since $v=0$ at the end points of the motion. Thus the mass has moved through half of it's periodic motion. Thus the period will be twice this amount of time or $T=.5\;{\textrm s}$. The frequency is the reciprocal of the period, or 2 Hz.

Finally, the amplitude of the motion is the distance from equilibrium to on of the end points. Since the distance between end points is 36 cm, the amplitude is half this amount, or 18 cm.

Dan Cross 2006-10-18