This is a technique called triangulation. It's well known to those doing orienteering. By taking compass bearings to two landmarks, such as a mountain peak and a church spire, and then plotting this on their map, they can find out exactly where they are. This has also been used in the past during conflicts to find enemy positions by listening out for signals from two different spots. It could be that this is what has happened in our maths example. Radio signals for the ship could be being received at Dublin and Aberdaron, thus giving the location of the ship. Here we use trigonometry, understanding of compass bearings and knowledge of how to use a protractor to find the location of a ship.