How do I know which out of the Sine and Cosine rule I should use?

If you look at the formulae for the Sine and Cosine rules you'll see that the Sine rule involves two angles and two sides and the Cosine rule involves one angle and three sides.So have a look at the problem you're given and see how many angles and sides you are given and which ones you're trying to work out:-if you're given: a) two sides and an angle (you're trying to work out the second angle) or b) two angles and a side (you're trying to work out a second side) then use the Sine rule.-if you're given: a) three sides (you're trying to work out an angle) or b) two sides and an angle (yo're trying to work out the third side) then it's the Cosine rule.