What you're trying to find is the logarithm, not the power.

For 8^2 = 64, 8 is the base, 64 is the power and 2 is the exponent. The exponent equals the logarithm of the power as long as you don't change the base.

Use ln for base e (Euler's number, approx. 2.718) or log for base 10.

For example: 10^x = 1000. To find x, just do:

log(1000)[/code:3jalj99k]
...which gives you 3.
If the base isn't 10 or [i]e[/i], use this instead: [code:3jalj99k]log(power) / log(base)[/code:3jalj99k]
(You can use [b]ln[/b] instead of [b]log[/b], the result should be the same.)
Using the example above, 8^2 = 64, you get [code:3jalj99k]log(64) / log(8)[/code:3jalj99k]
...and that should give you 2.