could anyone help me with this problem please?

Prove that the definition of continuous function f: R to R is equivalent to the property that for any open set A in R, the inverse-image of A is open.

So I know the definition of a continuous function is that if for every x sub zero in R,and for every epsilon greater than 0, there exists delta such that the absolute value of f(x)-f(x_0) < epsilon if the absolute value of x-x_0 < delta.

I don't know what kind of open set would have the inverse image is also open.

I don't know how to approach this problem