Question 13185
You are doing good so far, except for one little mistake.  If the length is twice the width, then {{{L = 2W}}}, which changes the second step of your answer to {{{120 = 2(2W) + 2W}}}.  This will actually make the problem easier to solve...

Multiply 2 by 2W, and your equation is now {{{120 = 4W + 2W}}}.  The step you got lost on is to add the like terms.  Since 4W and 2W both have a W on the end, you can add them together to get {{{120 = 6W}}}.  Now solve for W, and you get W=20.  Plug this in to L=2W, and you get that L=40.