Using [this wolfram alpha link](http://www.wolframalpha.com/input/?i=limit%20%282%5Ex-7%29%5E%281%2F%283-x%29%29), I found that
\begin{equation}
\lim\_{x \to 3} (2^x - 7)^{1/(3-x)} = \frac{1}{256}.
\end{equation}

How can i come up with that limit (1/256) ? How to convert this expression ?