given 18 feet of fencing what is the largest area that you can enclose with the fencing.

The largest area a given perimeter can enclose is a circle.

The largest area of a rectangle for a given perimeter is a square.
The question did not specify that the area be a polygon, so the circle gives the largest area.
2πr=18
r=18/2π=9/π
Therefore largest area
=πr²
=81/π