Question

How do you find a missing side leingth of two triangles?

Answer 1

two triangles is a vague statement; a concrete example would help more

Answer 2

Let's say sides AB,BC of ABC and DE, EF of DEF are given. Then you draw AB and EF so that A = D and B and E lie on the same line and in the same direction of A/D. Next you draw a circle around B with radius BC and one around E with radius EF. The points where those two circles meat are the points you can choose for C / F.