Geometry question!?!?!?

Question

I need help with this question!!!?
Please explain.........
You discovered that knowing the side lengths of a triangle tells you exactly what its shape must be. You also found that the side lengths alone are not enough to tell you the shape of a quadrilateral.
How do these facts explain why braces are often used to help rectangular structures hold their shape?
[If you forgot what a brace is, here is a picture:
http://www.traditionaltimber.co.uk/doors_files/pinedoorweb.jpg
(The diagonal wood strips are the braces)]
Thanks!!!!!!!!!!!!!!!

Answer 1

We discovered ?