A:C
Step-by-step explanation:
Answer:

Hope this helps! If you need an explanation, I've got you.
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
Step-by-step explanation:
2 + x + 2x = 14
3x + 2 = 14 (combine like terms)
3x = 12 (subtract 2 from both sides)
Hence the answer is 3x = 12. (B)