Dont know what part

Hey there. This is called a pivot shoe. We'd be happy to help you find the right replacement, but we'll need to see a picture of the pivot shoe out of the window. We will also need the exact width and thickness of the shoe. From there we should be able to narrow down possible replacements. See the video below to learn how to remove the pivot shoe from the window. Also keep in mind that the pivot shoe is what enables the 'pivoting' of the window, but what enables the vertical movement of the window sash is your window's balances, which are likely tube-shaped mechanisms that are above and attach to the pviot shoe. It's worth checking to see if these need replacing as well.