Physics calculation- speed of a star?

Physics calculation- does anyone understand the solution to this question?
Physics calculation- speed of a moving star—?
The yellow line emitted by the helium discharge tube in the laboratory has a wavelenght of 587 nm. The same yellow line in the helium spectrum of a star has a wavelenght of 590 nm.
What can you deduce about the motion of the star?
Calculate the speed of the moving star. (speed of light= 3.00×10^8 m s^-1)

This is the official solution but I dont follow it!! any explanations?? thanks :)
Sorry cant put the solution here but heres the link :)
http://www.examinations.ie/archive/markingschemes/2010/LC021ALP000EV.pdf (page 8 of the solutions!! thanks)