I can't center my actors with ExtendViewport, Libgdx

Question

I'm making a level select screen and I need use ExtendViewPort (I can't use Fit for other reasons), but this viewport moves my actors to the left, I need center them. I put X and Y position corresponding with my virtual screen size. This is my problem:

I do this things:

setViewport(new ExtendViewport(720, 1280));

@Override
public void resize(int width, int height) {
    levelsStage.getViewport().update(width, height, true);
}

actor.setPosition(10,100); (Examples..)

Sry for my bad english, I speak spanish.

Answer 1

I already solved my problem using this:

viewport.getWorldWidth() 

I can't explain more because my english is so bad