ok, if it's just about whole powers that you want to take the n-th root of, then it's rather easy. (I actually jogged my own memory about that
)
like The_Dude said, just take 27^(1/3) power. Now 27 is 3³
so you get 3^((1/3)*3) equals 3¨1 equals 3.
but as for numbers that aren't true powers... No idea.