I'm really not entirely sure if this is the right forum for this question, so forgive me if it isn't.

Basically I've done this entire lesson, but this one problem in particular, there are parts of it I just don't understand the logic to. I've posted a picture of the question below here, and I'll also have a link with a picture of it incase the one I posted in this thread doesn't work for whatever reason:

mathproblem_zpsf3047d35.jpg Photo by Azuraiiii | Photobucket

So, in the question, I understand the math itself, what I don't understand is the logic to some parts. For example, I don't understand where they get the equation "48.5 - 2x" from. I Know they took the 48.5 as the total amount of fencing, but I just don't understand how they know to put that "2x" in there and why that works in this particular case, or how I would go about making that equation on my own if I didn't have the answer available to me.

Then, I also don't understand how they know to create the equation "y = x(48.5 - 2x)".

Just please keep in mind, I understand the purpose of what they're doing, I just don't understand how they know what to do, and when to do it.

Any help understanding how some of this works so that I could figure it out on my own would be greatly appreciated!

Thanks!