Answer:
14 ft
Step-by-step explanation:
After the sides are folded up, the depth of the box is 2 ft, so the area of the square bottom is ...
(200 ft^2)/(2 ft) = 100 ft^2
The edge dimension of the bottom is then ...
√(100 ft^2) = 10 ft
Each side adds 2+2=4 ft to the bottom dimension in each direction, so the original square piece of metal was 14 ft square.
