The triangle inequality theorem states that the sum of the lengths of any two sides of a triangle is greater than the length of the third side. (Hence, the sum of the two smaller sides is greater than the larger side)
This means that the third side is shorter than 6 and 9 combined, but also 9 is shorter than 6 and the third side combined.
If the third side's length is x, then 3<x<15.
