First find the number of marshmallows Jacob needs: 48 graham crackers / 2 graham crackers/marshmallow = <<48/2=24>>24 marshmallows
Then subtract the number of marshmallows Jacob already has from the number he needs to find the number he needs to buy: 24 marshmallows - 6 marshmallows = <<24-6=18>>18 marshmallows
#### 18