Question

A tray is moved horizontally back and forth in simple harmonic motion at a frequency of f = 1.68 Hz. On this tray is an empty cup. Obtain the coefficient of static friction between the tray and the cup, given that the cup begins slipping when the amplitude of the motion is 2.93 cm.

I used the formula A(2pif)^2/g and i got the answer 3.32E5
but i got it wrong. What did i do wrong