We only know that the area of the blanket is 12 square units. So the blanket could be 6x2, 3x4, 12x1, etc.
Each of those would have a different perimeter, and so would need a different amount of border material.
That’s what my reasoning was. Now I’m confused again. :(
Not only that, does the border overlap the existing perimeter or is it sewed onto the existing perimeter edge?