In the first problem, you are equating the area of the sheet of material, rather than the surface area of the box to the given area. The length and width of the sheet are not the same as the length and width of the box. You want to state:
Now use the condition on the length and the above constraint to get the objective function in one variable.
For the second problem, you are doing well...write the total area as a function of , then equate the first derivative to zero to find the critical value, and I suggest that you use the second derivative test to show this critical value minimizes the area.