The sine and cosine rule.

Formulae-

a/sin A = b/sin B = c/sin C - use this formulae to find a missing side.

sin A/a = sin B/b = sin C/c - use this formulae to find a missing angle.

To use the sine rule..

you need to know two sides and a non-included angle. (ASA)

two angles and a side. (SSA)

Cosine formulae -

to find out a missing side = a2 = b2 + c2 - 2bc cos A

to find a missing angle = cos A = b2 + c2 - a2/ 2bc

Use cosine rule when given two sides and an included angle (SAS) or 

three sides and want to work out the angle (SSS)