Answer:
x ≥ 1
Step-by-step explanation:
There are no domain restrictions on a(x), so we can find the domain of the composite function by looking at that composition:
The argument of the square root function must be non-negative, so we must have ...
3(x -1) ≥ 0
x -1 ≥ 0 . . . . . divide by 3
x ≥ 1 . . . . . . . add 1
The domain of b(a(x)) is x ≥ 1.