Question 178772
First off, draw a rectangular solid on a piece of paper. 
Label the sides L, W, and H. 
Now calculate the surface area. 
You should get contributions from the LW face of 2*LW, the LH face of 2*LH, and the WH face of 2*WH.
{{{SA=2LW+2LH+2WH}}}
{{{SA=2(LW+LH+WH)}}}
Now to the measurements,
Length = 3*Width=3*W, {{{L=3W}}}
Width = W 
Height=Width-6=W-6, {{{H=W-6}}}
Go back to the surface area equation and substitute,
{{{SA=2(LW+LH+WH)}}}
{{{SA=2(3W*W+3W(W-6)+W(W-6))}}}
{{{SA=2(3W^2+3W^2-18W+W^2-6W))}}}
{{{SA=2(7W^2-24W))}}}
{{{SA=14W^2-48W)}}}