Mysterious Cryptogram

You received a new cryptogram from Graham. The cryptogram is [ 5 15 24 8 / 9 12 21 8 / 18 17 / 23 24 21 23 15 8 / 12 22 15 4 17 7 / 23 11 12 21 7 / 9 15 18 18 21 ] [Key : 3]

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