Vertical Oscillation assuming no damping

[xelzi]

Homework Statement

An 85-kg person steps into an experimental car of mass 300 kg, causing it to sink 4.5 cm on its springs. If started into vertical oscillation, and assuming no damping, at what frequency will the car and passenger vibrate on these springs.

Homework Equations

ω=√k/m
ω=2πf
x(t)=Acos(ωt)

The Attempt at a Solution

I thought 4.5 would be the amplitude of the oscillation but then I had 3 unknowns, the distance traveled, omega, and the time. Then I thought it may be the distance traveled and the amplitude would be 4.5/2 but I ended up stuck again.

 

[voko]

The amount of sink and the mass of the person are related via the stiffness of the springs.