6w-10 represents the total amount of fence will need to complete the rectangle of fence in his yard. The fence will need two lengths and 2 widths to complete the rectangle. If the width is w, and the length is 2w-5, then the total for the rectangle would be w+w+2w-5+2w-5 which simplifies to 6w-10.
If the width was w, then the length of the rectangular fencing would be 2w - 5 as he wants it 5 feet less than twice the width. So the total perimeter of the fencing would be w + w + 2w - 5 + 2w - 5 = 6w - 10
