Here is the walkthrough to Dooors 5 Level 21
Dooors 5 Level 21 – Tap the Wooden Box on the left Corner. Collect the SlingShot. Tap the Box Again and Collect Packet of Screwballs. Watch the yellow Cylinders and the number of Rings in it. Arrange the Squres According to the Numbers of the Rings as 3-7-5-3. Use the SlingShot to the Aiming Board. When the Indicator Gets Fully Green the Door Will Open and Escape.