Answer: 5cm.
Step-by-step explanation: The perimeter is all of the sides added up. We know that one side of the rectangle (the short side) is 3.5 cm. We have to multiply 3.5 cm by two, because there are two short sides. This gives us 7 cm.
Then we subtract 7 cm from 17 cm, and we get 10 cm. From 10 cm, we have to half it, because those are two of the lengths. Half of ten is five, so the length is five.