im not sure what you mean by y=3/x if you mean f(x)=3/x you need to remember f(x) is the function of 3/4 f(x) does not necessarily mean y x or any other its a way of saying this problem now if your wondering what f(x)= 3/x you only need know that a denominator can n ever b 0 so any real number EXCEPT 0 can be x. thats your solution