Rectangular Garden Area Calculation

Sarah wants to build a rectangular garden in her backyard. She has 40 feet of fencing to use. If she wants the length of the garden to be twice as long as the width, what will be the area of the garden in square feet?

Let the width of the garden be represented as $w$. Then, the length can be expressed as $2w$. The perimeter of a rectangle is given by the formula $$P = 2(l + w)$$, where $l$ is the length and $w$ is the width. Set up the equation based on the available fencing.