First find the value of the 2 cards Matt trades: $6/card * 2 cards = $<<6*2=12>>12
Then find the total value of the 3 $2 cards: 3 cards * $2/card = $<<3*2=6>>6
Then subtract the value of the cards Marcus gave up from the value of the cards he gained to find his profit: $9 + $6 - $12 = $<<9+6-12=3>>3
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