Define variables and set up the perimeter equation.

Let the length of the side with the stone wall be $x$ yards.
Let the other dimension (width) of the rectangular enclosure be $W$ yards.
The fencing is used on the remaining three sides: the two sides of length $W$ and one side of length $x$.
The total length of fencing used is 100 yards.
So, the equation for the total fencing is:

$$2W + x = 100$$

From the fencing equation, we can isolate $W$:

$$2W = 100 - x$$ $$W = \frac{100 - x}{2}$$

The area $A$ of a rectangular enclosure is given by the product of its length and width. In this case, the dimensions are $x$ and $W$.

$$A = x \cdot W$$

Substitute $W = \frac{100 - x}{2}$ into the area formula to express $A$ as a function of $x$:

$$A(x) = x \left( \frac{100 - x}{2} \right)$$ $$A(x) = \frac{100x - x^2}{2}$$ $$A(x) = 50x - \frac{1}{2}x^2$$

The area of the enclosure as a function of $x$ is: