Answer:
4 1/4 inches
Step-by-step explanation:
First assume that the page is square, so that all of the four margins around the photo are of equal width.
Subtract the photo width (3 1/2 inches) from the page width (12 inches); you'll get 8 1/2 inches. Then divide this 8 1/2 inches by 2: 4 1/4 inches.
Position the photo so that it's surrounded by a 4 1/4 inch border all around.
