area of the shaded region = area of the big rectangle - area of the smallest rectangle
rectangle general area = length * width
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Replacing the given data
area of the shaded region = (x-1)(x+5) - (x+1)(x-4)
= (x^2 -x +5x -5) - (x^2 +x -4x -4)
= x^2 - x^2 + (-x +5x -x +4x ) + (-5 + 4)
= 0 + 7x -1
= 7x -1
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Answer
= 7x -1
