Let the length of the fold be x inches. Then the base will have length 12-2x inches. By using we can find the length of the folded side to maximize area.
a=-2 and b=12
Turn the edges up 3 inches.
A drain is to be made up of rectangular aluminum sheets 12 inches wide by turning up the side edges 90 degrees. What depth (of the edges) will provide a maximum cross sectional area and thereby provide for the greatest flow of water