Question 2172
logx81=4

we know logxa=k if a= x^k 

that means ...
if 'a' can be written as k th power of x then we say k is logarithm of a to base x.

as we see 3 = 3^1
9=3^2
27=3^3
81=3^4

which shows 81 can be got as 4th power of 3.  so, logx81=4 implies x=3