Calculate surface area?

[Dillinger4]

Am i doing this right?

The base of a closed rectangular box has a length equal to 3 times it's width, which is w inches; it's height is 6 inches less than it's width. Express the surface area ,S of the box as a function of w.

I had no clue how to calculate the surface area of a rectangle so i looked it up. I thought is was equal to volume v = lwh. Seems it's sa = 2(lw) + 2(lh) + 2(hw). So expressing the surface area S as a function of w should yield the following. sa(w) = 2(3w(w)) + 2(3w(w-6)) + 2(w-6(w)) which should boil down to S(w) = 14w² - 48w. Looks right? Thanks.

[opus]

I came to the same result

[Dillinger4]

Thanks opus.