If you buy this computer now, you’ll get something thrown in, so it’s a deal.
Michael's t-shirts have a lot of salad dressing stains on them.
What’s that thing attached to your earlobe?
You know that the word 付ける means "to attach" and you know that the kanji 付 means attach. So, this word probably has something to do with attaching. This is the intransitive version, though, where something is attached (and you're not doing the attaching). You can use the く as a hint. You see something attached on the wall. You say "Coo'"
The reading can be derived from 付ける, a word you should have seen a few levels ago. If you know that word's reading you should be able to figure out this word's reading as well (and vice versa).