Simple Doppler effect physics question please help please help please someone !! :((?

A hydrogen atom, when vigorously perturbed, can emit light with a frequency of 6.16 � 1014 Hz. When the same light from hydrogen atoms in a distant galaxy is observed on earth, the frequency is 5.71 � 1014 Hz. Calculate the speed at which the galaxy is receding from the earth (in units of the speed of light, c).