Answer: 26
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Work Shown:
w = width
12+w = length, because it's 12 ft longer compared to the width.
P = perimeter of rectangle
P = 2*(length + width)
P = 2*(12+w + w)
P = 2(2w+12)
P = 4w+24
The perimeter 4w+24 must be 128 or larger

Therefore, the width must be 26 feet or longer.
The smallest value you can use is w = 26
If w = 26 is the width, then 12+w = 12+26 = 38 is the length
P = 2*(L+W) = 2*(38+26) = 2*(64) = 128 is the perimeter
This helps confirm the answer.