Answer
-4
Work Step by Step
Based on the definition of the common logarithm, we know that $log(x)=log_{10}x$.
Furthermore, recall that $log_{b}x=y$ is equivalent to the statement $b^{y}=x$ (where $x\gt0$, $y$ is a real number, and $b\gt0$ and $b\ne1$).
Therefore, $log(.0001)=log_{10}.0001=-4$, because $10^{-4}=\frac{1}{10^{4}}=\frac{1}{10000}=.0001$.