this means that we have to didvide the 12 inches wide side into 3 parts: one of 3.5 inches, and two other, of equal size (the margins).
the margins together will have (12-3.5=8,5) inches, which means that each of them will hace half of this: so we have to divide 8.5 into 2.
this is: :
8.5/2=4.25
so he has to to put it 4.25 inches from each side.