Can i use 4gb 1rx8 pc4-2133p-sa0-11 sk hynix ram on my a515-43 or should it be 2400

There are some considerations you have to take into account, but if you have the system and the stick already the easiest thing to do would be to try. Many times compatibility is just trial and error. If it doesn't boot, doesn't detect the memory, etc. then you can't.

The maximum supported memory speed of the 3500U seems to be DDR4-2400, but when you add that one, if the one you have already has a profile with equal or similar timings to those of the new stick it will downclock to 2133 MHz and the system will work as if both were 2133 MHz. You may win anyway if part of the memory is double channel enabled.

You can find some info on RAM upgrades in this thread, in case you wish to delve more into the intricacies of getting two sticks working together: https://community.acer.com/en/discussion/608352/guide-how-to-find-out-if-you-can-upgrade-the-ram-and-which-one-you-need