\textbf{155.} Suppose the length of the radical axis of two circles with radii $1 - 6a$ and $6 - 2a^2$ equals 6 units. If the two circles have exactly one common tangent, what is the average of the possible values of $a$?
\begin{itemize}
\item[(1)] $3$
\item[(2)] $\dfrac{13}{3}$
\item[(3)] $6$
\item[(4)] $7$
\end{itemize}
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