line break in math mode

[Mike Bonhoff]

Hi all,

I'm wondering how to make a line break in the discharged math mode. I tried
it with Strg+Enter but then there where 3 new boxes in the new line. It
looked like the eqnarray-environment-boxes and it didn't work fine:

It should look similar to this:
f(x)={z\in Z:there is an y\in Y and functions g,h\in F
              such that g(x)=y and h(y)=z}

But at the moment (with Strg+Enter) it looks like
                       f(x)={z\in Z:there is an y\in Y and functions g,h\in
F
such that g(x)=y and h(y)=z}

Does anybody know how to do this.

Thanks!