New Super Mario Bros.

If you mean the banzai bill, duck under it, then charge up a run and jump over the huge cannon to the flag.

If you are asking for the secret exit of level 7-5, get to the end of the level just before bazani bill. There should be a pile of boxes, under it a pipe leading down. Jump on a bom-omb, take it, and put it on the third step either way up. Then, run for your life! Go and get one more bom-omb, sprint back to the demolition site, and place it on top of the blocks on top of the pipe. Run off, come back again, and go down the pipe, to the secret exit leading to level 7-6. If you wanted a walkthrough for the whole level. I highly advise you use the FAQs on GameFAQs. Good luck!