Answer
$f = 1.1~Hz$
Work Step by Step
We can find $k$:
$kx = mg sin \theta$
$k = \frac{mg sin \theta}{x}$
$k = \frac{(30,000~kg)(9.8~m/s^2) (sin 30^{\circ})}{0.15~m}$
$k = 980,000~N/m$
We can find the oscillation frequency:
$f = \frac{1}{2\pi} \sqrt{\frac{k}{m}}$
$f = \frac{1}{2\pi} \sqrt{\frac{980,000 ~N/m}{20,000~kg}}$
$f = 1.1~Hz$
