According to the nature of triangle, knowing the length of two sides and the included angle between them, the length of the third side can be calculated by cosine theorem. The expression of cosine theorem is as follows:

c？ = a？ + b？ - 2ab * cos(C)

Where a and b are known side lengths, c is the third side length to be found, and c is the known included angle.

Please note that if there are only two known side lengths or no angle information, we cannot accurately calculate the length of the third side. The calculation results need at least one included angle and two side lengths.