Question

How do you evaluate `log_3 81`?

Answer 1

The answer is `4` because `3^4=81`

The question that the `log_a x` function is posing is:

To what power do I have take `a` to get to `x`

So in your case the question is:

To what power do I take `3` to reach `81`?

Usually, you will be working with '10-logs' or 'e-logs'.
Those you can find on your calculator:

`log_10=log`-button and `log_e=ln`-button

If you use these buttons with shift or 2nd you get the opposite functions, the antilogs, marked as `10^x` and `e^x`