Question

The vertical asymptote of the function f(x) = 3 log(x + 3)

Answer 1

A function has a vertical asymptote where it is not defined, and the logarithm is defined only if its argument is strictly positive: it has a vertical asymptote if its argument is zero, and is not defined if its argument is negative.

So, you have a vertical asymptote where

$x+3=0 \iff x=-3$