Question 1157082
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The original dimensions were  W inches wide and  (W + 6) inches long.


After making the box, dimensions of its base are  (W-2*3) wide and ((W+6)-2*3) long.


The volume is 3*(W-6)*W, and the equation to find W is


    3W*(W-6) = 336,   or

    W^2 - 6W - 112 = 0.

    (W-14)*(W+8) = 0

     W = 14.


<U>ANSWER</U>.  The original dimensions were  14 by 20 inches.
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Solved.