If the two Evidence cards are the 5 of Spades and the 7 of Hearts, the Murder card would be the 3 of Clubs (5 + 7 = 12, 12 – 9 = 3, and Clubs is the third suit).
If instead the two Evidence cards were the Ace of Hearts and the 8 of Hearts, the Murder card would be the 9 of Hearts (1 + 8 = 9, don't subtract anything because the sum isn't greater than nine, and the suit is Hearts because both Evidence cards are Hearts).