As per convention:
- the angle at A is named A, the angle at B is named B, and the angle at C is named C
- the side of leght a is the side opposed to A, the side of length b is opposed to B, and the side of length c is opposed to C.
Now just state the law of cosines:
c^2 = a^2 + b^2 - 2ab cos(C)
Also, a^2 = c^2 + b^2 - 2bc cos (A)
And, b^2 = a^2 + c^2 - 2ac cos (B)
