Let w be the frame uniform width. Then your setup equation is this= . To solve it, cross multiply 6*(10.5 + 2w) = 5*(14 + 2w). The rest is several easy steps 63 + 12w = 70 + 10w, 12w - 10w = 70 - 63 2w = 7 w = 7/2 = 3.5 inches. ANSWER Then the outer dimensions of the frame are 10.5+2*3.5 = 17.5 inches and 14+2*3.5 = 21 inches. ANSWER CHECK. The ratio of outer dimensions of the frame is = . ! Correct !