Answer
The value of $\nu_0$ is $5.531\times10^{14}s^{-1}$.
Work Step by Step
$s^{-1}$ is the unit of frequency here. So, this question in fact asks for the frequency $(\nu)$ of the light with $\lambda=542nm=5.42\times10^{-7}m$
The frequency of that light is calculated according to the formula $$\nu=\frac{c}{\lambda}$$(c is the speed of light in a vacuum, $c\approx2.998\times10^8m/s$)$$\nu=\frac{2.998\times10^8m/s}{5.42\times10^{-7}m}$$$$\nu\approx5.531\times10^{14}s^{-1}$$
So the value of $\nu_0$ is $5.531\times10^{14}s^{-1}$.
