The answer is:  "12 feet" .
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Note:  In a square, the length of EACH of the four sides of the square is the same.
Area = Length * width. 
For a square, length = width.
So for a square, Area = length * width = (length of a side)² = s² ,
Given:  A = s² = 144 ft² ;
Solve for the positive value of "s" .
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  →  s² = 144 ft² ;  Take the "square root" of each side ;
  → √(s²) = √(144 ft²) ;
  →   s = 12 ft.
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The answer is:  12 ft.
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