Its not clear if you want to show that any utility function with that represents the preferences must have the specified properties, or you just want to show that one exists.
Ill assume you only want to show that one exists. The simplest way to do this is to just identify a valid , and show that it represents the preference relation. There are many possibilities, but ill use .
its clear that as required.
Now you only have to show that represents your preference relation.
You're told that u represents the preferences, so it must be true that:
You need to show that
But you already know u() represents the preferences, so its sufficient to show that is the same ordering as u().
This is clearly true as u(0) is constant.
If you want to show rigorously that the above condition is sufficient to show the preferences are represented by then;
The final property that you must show is that if . This is straightforward from the definition of and the monotonicity of u(x).