Question 777841
The two equal sized top and
bottom border will equal 'x' apiece.
The long side border will = 48 - 2x
This is because we have taken the top and bottom 
borders out of the total.
Area of a rectangle = length * width
Area = (48 - 2x)* x
Area(x) = 48x - 2x^2
Differentiate
Area'(x) = 48 - 4x
Area'(x) = 0
48 - 4x = 0
-4x = -48
4x = 48
x = 12,
Width = 12 feet
Length = 24 feet
Area = 288 feet^2
Hope this helps.
:-)