I also know that the largest possible area of a rectangle with a fixed perimeter occurs when the rectangle is a square. So I want to find the side length of a square with perimeter 48. I can divide 48 by 4 to get the side length, which is 12. Therefore, the largest possible area of the rectangle is 12 times 12, which is 144. 
The perimeter of a rectangle is 48.  What is the largest possible area of the rectangle?