So, if the page is 12 inches wide, that's the width of the page, 12, minus the width of the photo, 7 1/2. 12 - 7 1/2 = 4 1/2.
Now, we need to divide 4 1/2 by 2, so it will be even on both sides.
4 1/2 divided by 2 = 2 1/4.
He should put the picture 2 1/4 inches from each side of the page.