Hey guys, how do i solve this problem?

Suppose f: R->R is continuous and let R. Prove that {x: f(x) not equal to is an open set.

My second midterm is coming up and i'm trying to study for it but i'm having trouble with some of the problems. i feel that when I see it i can learn it. Thanks guys (ps there will probably a few, considering how many questions are on this review sheet )