The length of the wire = 40 feet, which enclosed 2 sides (length and width)
Then, length + width = 40
The area of the rectangle = length x width
Let the width = x and the length = y, then
x + y = 40 and x * y = ?
We will use the first equation to find x in terms of y
Subtract y from both sides
x + y - y = 40 - y
x = 40 - y
Now the area will be the product of y and (40 - y)
If y = x, then each one will be 40/2 = 20
Then the area = 20 x 20 = 400 feet^2