How to find an angle of triangle that is not right angled when two sides are given

The Law of Cosines (also called the Cosine Rule) says:
c^2 = a^2 + b^2− 2ab cos(C)
The Law of Cosines is useful for finding:

the third side of a triangle when we know two sides and the angle between them

the angles of a triangle when we know all three sides 

The Law of Sines (or Sine Rule) is very useful for solving triangles:
a/sin A = b/sin B = c/sin C
a, b and c are sides.
A, B and C are angles.

(Side a faces angle A, 
side b faces angle B and 
side c faces angle C).

When we divide side a by the sine of angle A 
it is equal to side b divided by the sine of angle B, 
and also equal to side c divided by the sine of angle C