To reach the secret exit in World 7-4, you will need a mini-Mushroom. Go past the mid-level point and upwards to the left side of the screen. You will eventually see a flying "?" block. Use the mini-Mushroom to make Mario smaller and jump onto the block to stop it. Then, use the block to jump into the little opening in the brick wall. Make a "C" jump (backwards C) and jump up and to the left to land at the top of the wall. Go over and you will see a small pipe that only mini-Mario can fit in. Enter the pipe to reach the red flag secret exit.