The expression log 3 (x-4) is undefined for all values such that

Translate it

remember that translates to
so we can say it equals y
so
translates to

we want to find the values of x that make it undefined
well, we know that must be greater than 0
so we do
it is defined for
x-4>0
add 4
x>4

it is defined for all values such that x>4