jmc

Since the diameter of a circle is twice its radius, we know that $3x+5=2(x+6)$, or $3x+5=2x+12$. Subtracting $2x+5$ from both sides gives $x=7$. The circumference of a circle is $\pi$ times its diameter, so the circumference is $(3x+5)\pi =(3\cdot 7+5)\pi =(21+5)\pi =\boxed{26\pi }$.