Answer
The wavelength measured by an observer on the Earth is $864~nm$
Work Step by Step
Let $\lambda_0 = 480~nm$ and let $\lambda'$ be the wavelength measured by an observer on the Earth.
We can find the wavelength measured by an observer on the Earth:
$\frac{\lambda'-\lambda_0}{\lambda_0} = \frac{v}{c}$
$\lambda'-\lambda_0 = \frac{v}{c}~\lambda_0$
$\lambda' = \frac{v}{c}~\lambda_0+\lambda_0$
$\lambda' = \frac{2.4\times 10^8~m/s}{3.0\times 10^8~m/s}~(480~nm)+(480~nm)$
$\lambda' = 864~nm$
The wavelength measured by an observer on the Earth is $864~nm$.
