Robert Louis Stevenson's "The Strange Case of Dr. Jekyll and Mr. Hyde" revolves around a good doctor whose alter ego is a murderous monster. While trying to curb the dominant streak of the alter ego, the doctor decides to commit su icide to stop the monster.
In the given passage, Mr. Utterson and Guest were pouring over the letter reportedly written by Mr. Hyde, the monster, addressed to his benefactor Dr. Jekyll. And the arrival of the dinner invitation led Guest, who was an expert in handwriting, to discover that the handwritings in the two pieces of paper were the same, with the only exception being the slanting pattern.
Thus, the prediction is that Guest will say the note addressed to Dr. Jekyll was written by Jekyll.
