CAT Exam # 3, Box Lengths

[gp]

I'm confused on how the radical is simplified. In my answer, I'm getting 3x. Can you show me step by step how to simplify?

Thanks.

[RonPurewal]

forum rules stipulate that you post the problem itself, with answer choices, when asking about it. please follow those rules by posting the question; thanks.

note that this is pretty easy to do, as you can cut and paste the thing in pretty much one shot.

thanks.