Three sided rectangle

The three sided rectangle formula made since to me.  The idea of how if you change the side lengths you end with a different area makes since because you are not multiplyimg ny the same numbers.  A=(P-2x)x  The (P-2x) part of the formula helps you find the length of the side by itself, this happens because of the fact you have three sides and you are subtracting the two sides that are the same length.  So if you made the sides that are the same length longer, the remaining side would be forced to be shorter.  This side shrinks at a two to one ratio in comparison to the other two sides.  This two to one ratio causes the area to change as well along with the length of the sides.  For example if I have a shape that has two congruent sides (same size) that are 5 meters and one side that is 10 meters by itself and the last side is one that is made by something else that does not mater because there is no limit to it, the perimiter of the three sides would be 20 meters and the area would be 50 meters squared.  With the perimiter of 20, if two sides are 6 meters long and one is 8 meters long, the area would be 48 meters squared.  Also with the same perimiter of 20 meters for the sides that we can control the length of, if we have two sides of 7 meters and one side of 6 meters, the area would be 42 meters squared.  In all of these examples for every time the length of "x" increased by 1 the length of (P-2x) decreased by two and this uneven change in varialbes causes the product to change.