It helps if you know that . Then

so x = -4.

To do this if you don't know that little fact:

<-- If you use or ln here then you don't have to use the change of base formula a little further on.

Now let's say you still haven't seen that . Use the change of base formula to change the base to e (or 10 if you desire):

So we have:

which you can plug into your calculator and get that x = -4.

-Dan