Harry decided that if anything would get himself on Slughorn's good side, was to turn in the meddlesome book. It had gotten him the highest marks of the year, but it was dangerous, even in his hands.
"Ah Harry, what a surprise! Come in, come in," he ushered Harry into the familiar stuffy office, "What can I do for you?"
Harry was almost surprised by how welcoming Slughorn was despite their previous spats about the memory topic. So he waded into the subject, "Professor…I wanted to give you this," he presented the worn and torn book.
"You're not giving up my class are you Harry?" he asked alarmed.
"No sir, it just…a previous student wrote in the margins. Some of it's very dark magic and I wanted to report it to you."
"Why…" his face drained of color, "why on earth would you give it to me?"
Harry sensed Slughorn was reliving the memory he wanted to forget. Harry played on that vulnerability, "So no one else will get hurt sir."
But Slughorn merely nodded and accepted the book. In the end he didn't reveal the memory, but as Harry was leaving he heard the distant sounds of paper being ripped and shredded.
USES POTION TO FINALLY FOUND OUT