Is this an enameled steel tub? Also, what have you tried so far besides bleach?

If this is an enameled steel tub, I would also try acetone, a hydrochloric acid based toilet bowl cleaner (but be careful using this around any chrome as hydrochloric acid will dissolve chrome plating), and oven cleaner.

You should be aware that you can make the tub LOOK nice again by having it refinished, but the refinishing coat is not nearly as hard and durable as the powder coating that was originally on the steel tub, and won't last nearly as long as the original powder coating would have.

If it wuz me, I would try those 3 items listed above on the spot near the back of the tub. Whatever takes it off will probably take of the spot near the drain. You have chrome near that front spot, so you'll have to do something to protect the chrome from the acid, perhaps cover the drain with plastic tape or something.

I suspect what you have are rough surface in those two areas, and soap scum has accumulated in the roughness and dirt from your feet has gotten embedded in that soap scum. If this is an enameled steel tub, oven cleaner won't harm the powder coating. I'd spray a bit of oven cleaner onto the stain at the back of the tub, let it work for a half hour, and then scrub that spot with an old toothbrush. (Don't use a natural bristle toothbrush; use one with plastic bristles. Oven cleaner and drain cleaner both dissolve animal hair.)