Once you know the positive value of $x$, substitute it into $y = 2x - 5$ to find the corresponding value of $y$.

After you have both $x$ and $y$, remember that $xy$ means the product of $x$ and $y$. Multiply them to get the final value.

In one expression line, type x^2 = 49. In another line, type y = 2x - 5. Desmos will show two vertical lines (for $x^2 = 49$ at $x = 7$ and $x = -7$) and a slanted line for $y = 2x - 5$.

Click the point where the line $y = 2x - 5$ intersects the vertical line with positive $x$. The coordinates of this point give the values of $x$ and $y$ that satisfy both equations with $x$ positive.

In a new expression line, type the product of the coordinates you found (for example, something like 7*9, using the actual $x$- and $y$-values from the intersection). The number Desmos displays is the value of $xy$.

From $x^2 = 49$, take the square root of both sides:

Because the problem states that $x$ is positive, we choose $x = 7$ and ignore $x = -7$. So $x = 7$.

Substitute $x = 7$ into the equation $y = 2x - 5$:

So $y = 9$ when $x = 7$.

Now multiply $x$ and $y$:

Therefore, $xy = 63$, which corresponds to choice C.