You can put this solution on YOUR website! L = 18x -12
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W = 6
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A = L*W
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A = (18x-12) *6
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A = 108x -72
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But unless you know x, you cannot determine what the area is.
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Let A = y, this makes the equation:
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y = 108x -72
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The area will be any point along the line, provided x > 2/3.