This question is dealing with phenomena of Redshift, where wavelengths are shifted due to movement between observer and event. The key information to extract from the question is we have a rest wavelength, an observed wavelength and want a velocity of the galaxy (relative to Earth) from where the light has come from. The observed wavelength is larger than the rest wavelength -> wavelength is increased meaning that the galaxy is moving away, hence the question asks for a recessional velocity . So we should expect a positive velocity as our answer.The formula for redshift is given in the higher formula sheet as *Z = (*λ_{obs}** - λ**_{rest}**) / λ**_{rest}Also given is **Z = V/c **, where c is the speed of light. We want to solve for V, as this is the recessional velocity, both equations are equal to Z, so we can equate, meaning :**V/c = (λ**