You can think of the domain of a function as all x-values for which the function is defined. here we see there is a problem at x = 3. if x = 3, the denominator becomes zero and the function undefined. thus the domain of this function is given by:

dom(f) = all real x where x not=3

or

dom(f) = {all x| x not=3}

or

dom(f): (-infinity,3)U(3,infinity)