Geometry problem about two externally touching circles Two circles of radius $~12~$ and $~3~$ touch externally. A line intersecting both of them intersects first circle at points $P$ and $Q$, second circle - at points $R$ and $S$. Three resulting line segments, two inside the circles and the one between them, are equal: $PQ=QR=RS$. Find their common length.
I have prepared a picture with Geogebra to illustrate

I was trying to solve this with no luck. 
After formulating it as a system of equations based on coordinates, with the origin being circles' common point and $X$-axis on the line connecting their centers, Wolfram Alpha helped me to find that the answer should be $\frac{3}{2}\sqrt{13}~$. 
Can you give any hints on how to solve this?
 A: Let $PQ=QR=RS=2x$, $AM$ and $O_2N$ be perpendiculars to $PS$ 
and $O_2K$ be a perpendicular to $AM$. 
Thus, since $$KO_2=MN=x+2x+x=4x,$$ $$AK=\sqrt{12^2-x^2}-\sqrt{3^2-x^2}$$ and $$AO_2=12+3=15,$$ by the Pythagoras's theorem for $\Delta AO_2K$ we obtain:
$$(4x)^2+\left(\sqrt{12^2-x^2}-\sqrt{3^2-x^2}\right)^2=15^2.$$
Can you end it now?
I got $PQ=QR=RS=\frac{3\sqrt{13}}{2}.$
A: Let $PQ=QR=RS=2\ell$.
Drop a perpendicular from center $A$ onto chord $PQ$, let that point be $J$. Similarly drop a perpendicular from the center $O_2$ onto chord $RS$ and call it $K$.
Consider the right $\triangle AJP$, we have
$$AJ=\sqrt{144-\ell^2}.$$
Likewise in  right $\triangle O_2KR$, we have
$$O_2K=\sqrt{9-\ell^2}.$$
Now draw the line $O_2T$ which is parallel to $JK$ (same as saying parallel to the line intersecting the two circles). Then $O_2KJT$ forms a rectangle. Now
$$AT=AJ-TJ=AJ-O_2K=\sqrt{144-\ell^2}-\sqrt{9-\ell^2}.$$
Consider the right $\triangle ATO_2$. Observe that $AO_2=12+3=15$ and $O_2T=JQ+QR+RK=4\ell$. So
$$AT^2+O_2T^2=AO_2^2 \implies \left[\sqrt{144-\ell^2}-\sqrt{9-\ell^2}\right]^2+(4\ell)^2=15^2.$$
So we need to solve for $\ell$. 
Once you simplify this equation by appropriately squaring etc.  you end up getting
$$\ell^2(48\ell^2-351)=0.$$
This implies $\ell^2=\frac{117}{16}$. Consequently,
$$\color{red}{2\ell=\frac{3\sqrt{13}}{2}}.$$
