Answer
$f = 3.5~Hz$
Work Step by Step
We can find an expression for $\frac{k}{m}$ as:
$kx = mg$
$\frac{k}{m} = \frac{g}{x}$
We then find the frequency of oscillation;
$f = \frac{1}{2\pi}~\sqrt{\frac{k}{m}}$
$f = \frac{1}{2\pi}~\sqrt{\frac{g}{x}}$
$f = \frac{1}{2\pi}~\sqrt{\frac{9.80~m/s^2}{0.020~m}}$
$f = 3.5~Hz$
