Two patrol boats M3 and M7 leave the port at the same time. M3 heads due west and M7 heads on a bearing of 277degrees. After 30 minutes M7 has traveled 18km and observes M3 in a direction due south. How far is M3 from M7?
After you make your sketch you will see that
sin 7° = y/18, where y is the distance between them at that time
y = 18sin7° = ....