New Super Mario Bros. 2/World 2-A

This level largely takes place in the sky. Start by hopping onto the platform ahead. Jump off on the other side before it falls too far. Then, hit the ? block on the right to get a Super Leaf. Continue running left and jumping over koopas until you reach the midpoint flag. At the midpoint, hit the top-right ? block for another Super Leaf. Hop onto the series of platforms ahead and jump to the next platform in time until you reach a new patch of solid ground. Here, hit the ? block on the left for yet another Super Leaf. Take the last platform to the flagpole, following the coins to get on top.

Star Coins
For the first star coin, jump straight up on the top of the third platform after the coin roulette clock. You will land on another platform, which will take you up and to the left. There will be another platform to land on, which has the star coin on it. This platform will deposit you at a safe spot.

For the second star coin, grab onto the last koopa before the midpoint flag. Carry it over to the platform just to the left of the flag, and throw it towards the star coin out of reach. Its hitting the coin will grant it to you.

The third star coin is shortly after the midpoint flag. As the first platform reaches its zenith, jump onto the other one that appears. You will be able to jump along a series of platforms taking you higher and higher into the air. Eventually, one of them will shoot you to the right, where the star coin is floating midair.