Box optimization L, W, H, and domain

A box is to be made out of a 12 cm by 18 cm piece of cardboard. Squares of side length http://webwork.asu.edu/webwork2_file...dd0b8b8e91.png cm will be cut out of each corner, and then the ends and sides will be folded up to form a box with an open top. (a) Express the volume http://webwork.asu.edu/webwork2_file...850f274371.png of the box as a function of http://webwork.asu.edu/webwork2_file...dd0b8b8e91.png.

http://webwork.asu.edu/webwork2_file...f7e0799941.png http://webwork.asu.edu/webwork2_file...a3183e0181.png

(b) Give the domain of http://webwork.asu.edu/webwork2_file...850f274371.png in interval notation. (Use the fact that length and volume must be positive.)

(c) Find the length http://webwork.asu.edu/webwork2_file...6819fae171.png, width http://webwork.asu.edu/webwork2_file...e5b96d4b01.png, and height http://webwork.asu.edu/webwork2_file...6137056b01.png of the resulting box that maximizes the volume. (Assume that http://webwork.asu.edu/webwork2_file...48ec5da731.png).

http://webwork.asu.edu/webwork2_file...6819fae171.png = cm

http://webwork.asu.edu/webwork2_file...e5b96d4b01.png = cm

http://webwork.asu.edu/webwork2_file...6137056b01.png = cm

(d) The maximum volume of the box is http://webwork.asu.edu/webwork2_file...a3183e0181.png.

i found the equation and set the first derivative equal to zero and solved for x then plugging that number x into the original equation i found to get the max volume.

any help with finding the L, W, and H and the domain would be great.

thank you