a picture frame measures 13 in by 9 in. There is 88^2 of the picture showing. the frame is uniform in thickness. Find the thickness of the frame.
Assuming you mean 88 in, the total area of the painting is 13*9. The area shown is 88. If it is uniformly thick all around, we can subtract the thickness form the painting.
$\displaystyle (13-n)(9-n) = 88$ Solve the quadratic, and then divide n by 2, since we are counting for two sides combined (top and bottom or left and right)