Answer
$f=1.91\times 10^8Hz$
Work Step by Step
To find the wavelength, use the relation that $k=\frac{2\pi}{\lambda}$ to solve for $\lambda$, yielding $$\lambda=\frac{2\pi}{k}$$ Substituting a value of $k=4.00 m^{-1}$ yields $$\lambda=\frac{2\pi}{4.00m^{-1}}=1.57m$$ Using the relation that $v=f\lambda$ and that $v=c$, $f$ can be solved for. $$f=\frac{c}{\lambda}$$ Substituting the value of $\lambda=1.57m$ yields $$f=\frac{3.00\times 10^8m/s}{1.57m}=1.91\times 10^8Hz$$
