Finding the Domain and Range of a function composition

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I'm having trouble finding the domain and range of a function composition.

$f(x) = x^2 - 3x$

$g(x) = \sqrt{x}$

$(g \circ f)(x) = g(f(x)) = \sqrt{(x^2 - 3x)}$

How do I find the domain and range of $(g \circ f)(x)$?

(I know the answer because it's in the back of the book, but please tell me how?)

Thanks in advance!