Domain is the set of numbers for x we can plug in for this function .
Here x is inside redical sign but there it is " -x "
we know that sqrt of a negative number is an imaginary value
so we need to avoid such values of x which gives a negative value inside redeical .
the term inside redical should be equal or greater than 0 only
so -x >= 0
that means x <= 0
so domain is : all negative real numbers + 0
Answer : (- inifnity , 0]