1. Draw a sketch.

2. The complete area is calculated by:

3. The fencing consists of 2 lengthes and 5 width:

4. Plug in this term into the equation of the area to get the equation of the function:

5. The graph of the function is a parabola opening down with it's maximum value at it's vertex. Determine the coordinates of the vertex to get the value of l when the largest area occurs.

Spoiler: