If you are given two sides' lengths and an included angle whose sides/rays include the sides that have given measurements, you would use the law of cosines, which states that c^2=a^2+b^2-(2ab*cos(C))
If otherwise, you would use the Law of Sines, which states that sin(A)/a=sin(B)/b=sin(C)/c.
The letters mean the side opposite of their angle. For example, given triangle ABC, b would be the side opposite of angle B, which is line segment AC.