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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.
ANSWER. The original dimensions were 14 by 20 inches.
Solved.