Undamped, forced oscillation?!?!?

I have this question and can't seem to get the right answer.

Damping is negligible for a 0.130 kg object hanging from a light 6.30 N/m spring. A sinusoidal force with an amplitude of 1.70 N drives the system. At what frequency will the force make the object vibrate with an amplitude of 0.440 m?

I tried the equation A= +/- (f/m) / (W^2-Wo^2) and can't seem to figure out a certain step. Any help would be much appreciated! :)