A source emitting electromagnetic signals with proper frequency w0 = 3.01010 s1 moves at a constant velocity

Question:

A source emitting electromagnetic signals with proper frequency w0 = 3.0∙1010 s–1 moves at a constant velocity v = 0.80 c along a straight line separated from a stationary observer P by a distance l (Fig. 5.37). Find the frequency of the signals perceived by the observer at the moment when
(a) The source is at the point O;
(b) The observer sees it at the point O.