How do I know which rule of trigonometry to apply in a question?

There are four trigonometry rules which you need to know: Pythagoras, SohCahToa, the cosine rule and the sine rule. The first two only work on right angle triangles, and the second two apply to non-right angle triangles. Depending on whether the question asks you about a right angle or a non-right angle triangle. Then look at what the question gives you, and what it wants you to find. How many angles do you count? Zero angles means you use Pythagoras. One angle means you use SohCahToa or the cosine rule. Two angles means you use the sine rule.This seems like a lot to take in at first, but I promise it becomes a lot easier with practice. Once you know this, the most important thing is to try out lots of different questions. Soon you will become very good at knowing which rule to apply when.