Check out http://www.candycrush-cheats.com for guides, walkthroughs and cheats of every level of Candy Crush.
The goal of level 21 is to clear 27 jelly squares in 30 moves and and get at least 30,000 points.
http://candycrush-cheats.com/level-21/
Candy Crush Level 21
Candy Crush 21