Answer
$f=5.58Hz$
Work Step by Step
We know that
$a=-\omega^2x$
$\omega^2=-\frac{a}{x}$
taking square root on both sides
$\omega=\sqrt\frac{-a}{x}$
put the values
$\omega=\sqrt\frac{-(-123)}{0.100}=35.07\frac{rad}{s}$
As $\omega=2\pi$$f$
$f=\frac{w}{2\pi}$
$f=\frac{35.07}{2(3.14)}$
$f=5.58Hz$
