Question

log[x](16)=4

Answer 1

x = 2

By definition, this expresssion translates to x^4 = 16.

x^4 - 16 = 0
(x^2 +4) (x^2-4) = 0
(x ^2 +4) = 0 NO real roots
so, x^2 - 4 = 0 and x = 2 or -2.
-2 is exraneous leaving x = 2.