Answer
$m=4.3kg$
Work Step by Step
First, we find the spring constant $k$. The weight of the object pulls the spring down, so $$mg=F_x=kx$$ $$k=\frac{mg}{x}=\frac{2.8\times9.8}{0.018}=1524.44N/m$$
We know the frequency of vibration can be calculated by $$f=\frac{\omega}{2\pi}=\frac{1}{2\pi}\sqrt{\frac{k}{m}}$$ $$\sqrt{\frac{k}{m}}=2\pi f$$ $$m=\frac{k}{4\pi f^2}$$
We have $k=1524.44N/m$. To achieve frequency of vibration $f=3Hz$, the mass of the object needs to be $$m=4.3kg$$
