Since the dimensions of the length and width are equal, then the board and the picture are squares. The width of the board is the margin surrounding the picture. Let's denote the width to be x. The length of the picture plus the length of the width at both ends should equal the length of the board. So, the equation should be:
10 1/2 in = 8 3/4 + 2x
Solving for x
x = 7/8 inches