Homework Help: Need help with the 2nd part of a frequency/ relative speed quesiton

1. Jan 21, 2009

thundercats

1. The problem statement, all variables and given/known data

A galaxy known to be traveling away from the Earth at 0.650 percent of the speed of light is observed on Earth to have a strong peak in its spectrum at 649.1 nm. However, one small portion of the galaxy (a "dwarf") has a similar observed peak with a wavelength that is 4.50 nm shorter. (This situation is not as strange as it may sound. Several galaxies can be observed to be in the process of collision or merger, such as the Whirlpool Galaxy. Several others, particularly radio galaxies, often exhibit high-speed jets or lobes.)
1) Calculate the wavelength of the light emitted by the bulk of the galaxy.
i understand this part of the question and got this answer 644.9 nm
What is the speed of the dwarf, relative to its "parent" galaxy?

2. Relevant equations
F'=F(1(+or-)U/C)

3. The attempt at a solution
i dont understand part 2 i have got part A i just dont know where and how to start