Help installing rear derailer

[Bikin4Life]

Sram X4 derailer on Diamondback MTB broke off from the frame. Flat donut looking piece that attaches to frame with set screws broke in half. I don't see that piece when I look at replacement derailers. Is that piece separate from derailer? If so, what is it called so I can order it? Please educate me. Thanks!

[nfmisso]

pictures required.

[1FJEF]

Sounds like the derailleur hanger. Did it look like one of these?
The SRAM X.4 uses SRAM shifters with a 1:1 cable pull ratio. You need to replace your derailleur with another X.4 (inexpensive & good) if you want to keep your existing rear shifter. If you don't you'll be fubar, probably a new shifter, new cable.... If you're broke (like me, lol) you can save a few bucks with a SRAM X.3 RD.