If the perimeter is 322 cm and the width is 69 cm and the formula for perimeter is 2l+2w=perimeter then how to solve would be 69+69=138(you’re adding both widths) then take 322(perimeter) and subtract your width, 138, from it to get 184 which is the total of both your lengths added together so to get the total of just one length, take the number you got, 184, and divide it by two to get your answer that the length of the painting is 92.
Check: 69+69+92+92 and if it equals 322 you’re good! (I hope this helps, I’m no expert but I’m pretty certain this is correct)