The area of a rectangular wall of a barn is 36 square feet. its length is 6 feet longer than twice its width. find the length and width of the wall of the barn.

The area of a rectangle is A = length x width
You would therefore get 36 = length x width
Then, since length is 6 feet longer than twice its width, 
you would get length = 2(width) + 6
You would therefore end up with 36 = (2w + 6) x w
solving for width you find it to be 3 feet
Plugging that back in to the area formula, you get 12 feet to be the length.