Step-by-step explanation:
The text is in Romanian language, however here's a clearer rendering which reads;
"From 2 coherent sources at a distance d = 2 meters from each other, it propagates in an elastic medium where with velocity = 340 m/s. At a distance L = 4 meters from the middle between sources perpendicular to the segment joining them a maximum of interference and the next at a distance of 1.5 meters from it on the line parallel to the one on which the sources are located. What is the frequency of oscillation of the sources?."