The domain of a rational function - one with a variable in the denominator is the value of the variable that makes the fraction undefined.
in this case g(x) is the denominator and whatever makes it 0 or undefined is not allowed to be in the domain
sqrt(x) >0
if x = 0, the square root of 0 is 0 and the denominator is not allowed to be 0
If x<0 then the square root of x is not a real number
the domain is x>0 or (0, infinity)