Question 861078
The domain of the function is the set of values of the variable for which the function has a value.
{{{f(x)=sqrt(x-3)}}} can be calculated as a real number in all cases where
{{{x-3>=0}}}<--->{{{x>=3}}} .
If {{{x<3}}}<-->{{{x-3<0}}} you cannot calculate the square root as a real number.
So the domain of {{{f(x)=sqrt(x-3)}}} is all the real numbers that are greater than or equal to 3.
Maybe you are expected to write it as
{x | {{{x>=3}}} } or some such thing.
Maybe the notation expected is  {{{"["}}}{{{3}}}{{{","}}}{{{infinity}}}{{{")"}}} .