Let's discuss the idea behind trying to do this.
You have two points on a circle, $(a,b)$ and $(c,d)$. You know that the center of the circle, wherever it may be, must be equidistant to $(a,b)$ and $(c,d)$ (that is, the distance from the center to $(a,b)$ must be equal to the distance from the center to $(c,d)$).
However, any point that is equidistant to $(a,b)$ and $(c,d)$ will work as the center of some circle that goes through $(a,b)$ and $(c,d)$: if $(h,k)$ has distance $r$ to $(a,b)$ and $(c,d)$, then $(x-h)^2 + (y-k)^2 = r^2$ will be a circle with center $(h,k)$ that goes through $(a,b)$ and $(c,d)$.
What are all the points that are equidistant to $(a,b)$ and $(c,d)$? It is the line that is perpendicular to the segment that joins $(a,b)$ and $(c,d)$ and goes through the midpoint of that line; see for example here. It's known as the "perpendicular bisector".
The line segment from $(a,b)$ to $(c,d)$ is given by the parametric equations $\begin{align*} x &= a + t(c-a),\ y &= b + t(d-b), \end{align*}\qquad $0\leq t\leq 1,$ and the midpoint is given when $t=\frac{1}{2}$; that is, the midpoint is $\left(\frac{a+c}{2}, \frac{b+d}{2}\right).$ If $b=d$, then the segment is horizontal, so the perpendicular bisector is vertical with equation $x = \frac{a+c}{2}$. Any point on that line will be a center of a circle through those two points.
If $b\neq d$, then the slope of the perpendicular bisector is $m=-\frac{c-a}{d-b}$ (the negative reciprocal of the slope of the line through $(a,b)$ and $(c,d)$), so the equatio of the perpendicular bisector is $ y = -\frac{c-a}{d-b}\left(x - \frac{a+c}{2}\right) + \frac{b+d}{2}.$ Any point on that line is the center of a circle that goes through $(a,b)$ and $(c,d)$.
So there are infinitely many solutions to your two equations.
If you know the value of $r$, that reduces the possible solutions to $2$, except if one case: the only case where there is a unique solution is the case where the distance is exactly half the distance between $(a,b)$ and $(c,d)$; otherwise, you always have two solutions, that are symmetrically placed on both sides of the line segment through $(a,b)$ and $(c,d)$.
In the case you give, we have $(a,b) = (20.1, 17.94)$, $(c,d)=(3.25,15.81)$; so the center of any circle through those two points lies on the line $ y = -\frac{3.25-20.1}{15.81-17.92}\left( x - \frac{23.35}{2}\right) + \frac{33.75}{2} = \frac{16.85}{2.11}\left(x - 11.625\right) + 16.875.$ If you know that $r^2 = 285.27$, then this gives you two possible points on this line whose distance to $(a,b)$ and $(c,d)$ is $\sqrt{r}$. Just take $(x-a)^2 + (y-b)^2 = r^2$, plug in the values of $a$, $b$, $r^2$, substitute the value of $y$ for the expression on $x$ given above, and solve for $x$ (e.g., using the quadratic equation). That will give you the two values of $x$ that correspond to $h$, with the expression for $y$ giving the corresponding values of $k$.