I have a problem grasping how can I calculate following:

Given circle $A$ with radius $r\_A$ and center point $S\_A = (r\_A, r\_A)$ and circle B with radius $r\_B$ and center point $S\_B = (2r\_A, d)$ calculate $d$ knowing that circles intersect in one point only and $d$ is smallest possible.

And I'm stuck with even starting it. Tried drawing it, but got to a point I just created following set of equations:

$\begin{cases} (x - r\_A)^2 + (y - r\_A)^2 \mbox{=} {r\_A}^2 \\ (x - 2r\_A)^2 + (y - d)^2={r\_B}^2 \end{cases}$

This however gets me nowhere since I have 2 equations and 3 variables. What am I missing here?