Question 1046639
Area = length * width 
Let s = side of square.  A(square) = s^2
Square: 
Length: s Width: s Area: s^2 

Rectangle: 
Length: s + 12 Width: s - 4
 Area: (s + 5) (s - 4) 

area unchanged.
(s + 5) (s - 4) = s^2
s^2 + s - 20 = s^2
s = 20ft for the Square
25ft by 16ft the Rectangle

AND
400ft^2 = 25ft*16ft  CHECKS!