How to fix yellowing tub?

What is the tub made of?

https://www.doityourself.com/stry/how-to-whiten-a-bathtub

Don't use bleach on a fiberglass tub. It will yellow. Fill the tub's bottom with warm water and add 2 cups Borax. Use a broom and dissolve powder. Let sit overnight. Drain tub. Using spray of equal parts vinegar and warm water, clean with sponge. Good luck.

I have a fiberglass tub that was terrible when we bought the house. It had stains from what looked like self tanner. I didn’t want to replace it because it was a working whirlpool tub and I didn’t want the expense of replacing it. It was not stained when we looked at yhe house before we bought it , just dirty. I cleaned it with baking soda. I first used a damp rag and kept adding baking soda to it as I cleaned It. Then I got smart and bought one of those battery operated spinning brushes with a long handle , dampened the tub , put baking soda on the stains and scrubbed away. Be sure to rinse well. The stains came out and the tub looks shiny again like it was new. I do it about every other week now. It keeps it shiny and fresh.

I read the other Day peroxide is good

How can I clean a full SUPPOSED TO be white tiles

Easy Off No Fume Oven Cleaner in the BLUE can. Spray it on and let it liquefy. May need some scrubbing. Wear rubber gloves. Rinse well. Works on enamel, porcelain, and fiberglass.