First find the total amount of velvet James uses on the hats: 12 hats / 4 hats/yard = <<12/4=3>>3 yards
Then find the total amount of velvet he needs to make the cloaks: 6 cloaks * 3 yards/cloak = <<6*3=18>>18 yards
Then add those two amounts to find the total amount of velvet he needs: 3 yards + 18 yards = <<3+18=21>>21 yards
#### 21