Question

9/x - 6 Find all numbers that must be excluded from the domain of the rational expression.

Answer 1

for these always assume domain is All Reals
then look for 2 exceptions:
zero in denominator or negative in square root
In this case the x is in denominator, set expression = 0, solve for x
x-6 = 0
x = 6

Answer 2

i think that in the way the question has been put, only the 0 should be excluded, as division have precedence over subtraction, i.e. the expression is (9/x)-6, so it has no meaning with x = 0.