From the Super Mario Wiki
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This underground level contains Buzzy Beetles and Broozers. Mario must let a Broozer break some blocks to get the first Star Coin. After avoiding a few falling Buzzy Beetles, Mario should grab one of their shells and throw it at the second Star Coin, found past a moving platform. To get the third Star Coin (and an easier way to the flagpole), Mario must, after the midway point, see a row of four bricks with a longer row above it. He should jump on the bricks, hitting a Brick Block above the one on the far right. Then he should hit the brick he's standing on (the far right) to reveal a Beanstalk. The player should climb it to a secret area with a Broozer. Mario should hit the P Switch and let the Broozer punch his way through some blocks to get the third Star Coin. After he goes through the narrow hole, Mario will be at the flagpole.