If it's not a right angled triangle then chances are it involves one of these three rules that can be found at the bottom left hand corner of the Formulae Sheet.
If you are finding a side length or an angle and you can label side, angle, side (SAS) with the sides either side of the angle then it's the Cosine rule.
Choosing which rule to use is probably the hardest part for most students.
However you must ensure that a is opposite A, b is opposite B and c is opposite C.
The general advice I give is to ignore the letters that have been assigned to each corner in the question and to re-label the triangle making the angle you are trying to calculate the letter A or the side you are trying to calculate the letter a.
If you get the step above correct then it doesn't really matter which side you label b and which side you label c.
If you have two pairs of angles and sides e.g. a and A and b and B then you will need to Sine rule.
Confusing at first but practice makes perfect, a help video will be coming soon, in the mean time try to work your way through the practice questions below.