The perimeter is the sum of all sides therefore you will have two equal widths and two equal lenghs. If the width is 8 inches then both widths are 8+8 or 16 which leaves the two lenghs so If you take the total perimeter of 42 inches and subtract the total widths of 16 you have 26 inches left for the two lenghs. If you take 26 and divide by 2 you get a lengh of 13.