Question

F(x)=sqrt(9-3x) determine the domain of f

How would I break that out to determine f

Answer 1

I assume you mean the domain of x.

x has to be real, and has to be such that
9-3x>=0 or
x<=3

Answer 2

you are finding √(9-3x)

For this to result in a real number, 9-3x cannot be a negative number

so the domain is
9-3x ≥ 0
-3x ≥ -9
x ≤ 3