Question 511524
L = 2W +3
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L*W = 90
W = 90/L
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substitute
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L = 2(90/L) + 3
L = 180/L + 3
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L^2 = 180 + 3L
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L^2 -3L -180 = 0
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(L-15)(L+12) = 0
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L = 15 or L = -12
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Negative length is not possible, so 
L =15.
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Substitute 
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W = 90/L
W = 90/15
W = 6
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Check the area to be sure of the answer.

6*15 = 90
Correct.
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Answer: Length = 15; width = 6.
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Done.