If you are stuck on Candy Crush Level 340 you have to watch this. Pretty helpful walkthrough video:

Patricia Frederick’s Tips for Candy Crush Level 340

Use striped/wrapped combos on either side to release the isolated sections. Avoid the center isolated section of striped candies until you have less than 5 moves left (it’s a bomb dispenser.) Vertically striped candy matches or more striped/wrapped combos can move things along to help clear the jellies.