x and x - 12 represent lengths (assumed to be in cm)...and so must be positive. Lengths cannot be negative, and if we are to actually have two pieces...
Okay, you have a piece of wire whose length is let's say W...you bend it into a square...what is the perimeter of the square? Then using your formula...
Head of Mathematics Department, I.E.S. Sta Teresa, Madrid.
Prof. of Mathematical Methods, Univ. Alfonso X El Sabio, Madrid.
(Up until the 2008/2009 academic year. I am now devoted to investigation).