Which are the possible side lengths of a triangle?

If you know you have a triangle, then the sum of the shortest 2 sides must be greater than the length of the longest side.  Graph this for a better understanding.

This stems from the Pythagorean Theorem, which applies only to right triangles:  x^2 + y^2 = h^2, where x and y are the shortest two sides and h is the hypotenuse.

Don't know whether or not you have studied the Law of Sines and/or the Law of Cosines, but both are relevant here.

Main idea on left, details on right