How can I get my bath tub super white again? And my sink super shiny?

It is plastic so re enamelling would not be the answer. It seems to have gone slightly yellowish? What products should I use to get it back to very white?

Also, how do you get your kitchen sink really shiny?

What pretty things have you got in your bathroom? I have essential oils, beautiful coloured bottles, a really funky silver curtain plus soft puffy towels!