Hello, Jerry99!

The cardboard looks like this:

Code:

: - - - - 2x- - - - - :
- *---*-----------*---* -
: |///: :///| 3
: * - *-----------* - * -
: | | | | :
x | | | | x-6
: | | | | :
: * - *-----------* - | -
: |///: :///| 3
- *---*-----------* - * -
: 3 : - 2x-6 - : 3 :

The box looks like this:

Code:

*-----------*
/| /|
/ * - - - - / *
/ / / /
*-----------* / x-6
3 | |/
*-----------*
2x-6

The volume is 3300 cm

So we have: .

. . which simplfies to: .

. . which factors: .

. . and has the positive root: .