which part do you think they did not send?
the only part that you need is the new brake. everything else used is from your p3. the brake mounts to a black "adapter" using a 5mm bolt, the adapter mounts to the frame using a 4mm bolt (or reverse, can't remember). both must be removed to install a new brake.
ETA - this sounds like a good thing for you to take to your LBS if you can't figure it out. it's not difficult, and rear brakes are important.
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