When I find a problem like this, I find it helpful to think about what I know and what equation will help me.
The question tells us the frequency of the wave [tex](5 \times 10^{-9})[/tex] Hz. We want to work out the wavelength. What equation links these two quantities?
Wave Speed = Frequency x Wavelength
(we know all electromagnetic waves travel at the speed of light in a vacuum)
[tex](3 \times 10^8) = (5 \times 10^{-9}) \times \lambda[/tex]
and then divide by [tex](5 \times 10^{-9})[/tex] to get the wavelength.
Wavelength = [tex]6 \times 10^{16}[/tex] m
