Please solve

The length and width of a cardboard are 30 ft and 13 ft. We are going to cut out the corners and fold up the sides to form a box. Determine the height of the box that will give maximum volume. Note: Write only the numerical value of the height, the units are already written. Write the exact answer not the decimal approximation (for example write not 0.8). Also, if the answer is 3+15 then write 3+V5 or (3+sqrt(5))/4. A Answer: The height of the box that will give maximum volume is sin (a)