if two opposite sides are 4cm each then their total length is 8cm. If the total length is 24cm then to work out the possible lengths of the other two sides you subtract 8 from 24 which gives you 16. A rule in rectangles is that opposite sides are equal so each remaining side would have a length of 16/2 which is 8. To get all the possible answers you have to go through all the possible perimeters (assuming all perimeters have to be whole numbers) between 24(max) and 9(min) and do those steps.