If all the sides add up to 26 and one side has the length of 5, then you should know that there are 2 sides have the same length so what you have to do is draw a rectangle. you have two sides that are shorter than the other two sides. If you mark one side 5 cm( its best to use the shorter side for this problem- you will see why later) the one across from it should be 5 also. So then you subtract a total of ten from 26. 5+5=10 both sides equal 10, but the perimeter is 26 in total so you then subtract. 26-10=16. 16 is the total of both of the other sides put together so divide 16 by 2, 16/2 =8---> label the short sides 5 and the longer sides 8.