The piece is made out of two pieces of parachute, a short one and a long one. The short on is 21 meters shorter than the longer one, if the longer one is 36 meters than the short piece is 36 meters - 21 meters
36 - 21 = 15
This means that the short piece is 15 meters long
We can check this by adding the short piece and the long piece hoping that it equals the total length, 51 meters
