Log (base3) 81 = x
One of the log identities is
Log(base x) x = 1
We can rewrite 81 in terms of 3 so that we can use the identity.
Log (base 3) 3^4 = x
The power rule lets us bring the 4 in front as a coefficient so,
4* log (base3) 3 = 81
4 * 1 = x
4 = x