What's to know the rules in solving mathematical bearing questions

Well, the rules you'll frequently have to use are the cosine and the sine rules.

For a triangle of sides A, B and C, with angles opposite to side A being a, that opposite to side B being b and the same for c.

The sine rule is:
\frac{sin (a)}{A} = \frac{sin (b)}{B} = \frac{sin (c)}{C}

The cosine rule is:

A^2 = B^2 + C^2 - 2BC cos(a)