Knowing the length of 2 sides is not enough to nail down the length of the third side. There are an infinite number of possibilities that it could be.
Here's what you have now: You have two sticks. One of them is 6-ft long, and the other one is 19-ft long, and they're joined together at one end with a hinge. You can open them wide, or you can close them down to a narrow slit. Whatever you do with them, you can always join their other ends with another stick. It may be short, or it may have to be long. Knowing the lengths of the first two sticks doesn't tell you the length of the stick you need in order to make a triangle.
(So don't feel too bad that you don't know how to get the third side. There isn't a way.)
