Answer
$f = 0.692~Hz$
Work Step by Step
We can find the angular frequency.
$-\omega^2~x = a$
$\omega = \sqrt{\frac{-a}{x}}$
$\omega = \sqrt{\frac{-(-5.30~m/s^2)}{0.280~m}}$
$\omega = 4.35~rad/s$
We can find the frequency of the motion.
$f = \frac{\omega}{2\pi}$
$f = \frac{4.35~rad/s}{2\pi}$
$f = 0.692~Hz$
