3 square in a circle (geometric question) There is three square in a circle like the picture attached.
  How can find radius of circle  ?
I know that's not a hard problem,but I was away from geometry for years.  If possible give me hint or idea to start to solving.   Thanks in advance.

the smallest side is $6$
and medium square side is $6+6$
and big side is $6+6+6$
But I got stuck here and not to go over there ...

The circumradius of a triangle is
$$
R=\frac{abc}{4A}
$$
where $a,b,c$ are the sides of the triangle and $A$ is its area.

The sides are $18$, $\sqrt{36^2+6^2}=6\sqrt{37}$, and $\sqrt{36^2+12^2}=12\sqrt{10}$ and the area is $\frac12\,18\cdot36=324$. Therefore, the circumradius is
$$
R=\frac{18\cdot6\sqrt{37}\cdot12\sqrt{10}}{4\cdot324}=\sqrt{370}
$$
