"log" is not a variable. Just like you don't say "sin^2". It has to be the sin OF something. Similarly, you have to take the LOGARITHM of something.
Recall the definition:
The log operator takes the form of and allows you to solve for the exponent c if you have a and b. Look at the definition and see how it works here.
And as blair_alane said, where did the negative come from?