(a) Find where .

(b) State the domain of .

(c) Find .

The first question is easy to answer: . However, part (b) is what puzzles me. Without actually finding (since that is asked for in (c)), I am supposed to discover the domain of it. In the markscheme the answer is simply stated, without explanation, and the question is not worth many marks. So it seems as if I am missing something very obvious here, doesn't it?

According to the key, the answer is supposed to be . How is this reached in a - supposedly - trivial way?