Plot sqrt(3x/(1-3x)) to see that it is real valued only for 0 ≤ x ≤ 1/3 and it takes values between 0 and 1 only for x ≤ 1/6.
arccos(x) is defined only for -1 ≤ x ≤ 1
therefore, the domain of the given function is 0 ≤ x ≤ 1/6; the range is 0 ≤ y ≤ π/2
I think I will leave the rest of the problem for you to work out.