ds super mario bros

I assume that you are talking about New Super Mario Bros.. If I want to help you with the level, then I will help you.
To get the coin, you have to go to the end of the level and sink into the quicksand. Keep moving right and jumping occasionally so you can see where you are. When you are at the end of the quicksand, move right and you will instantly warp underneath the area with the flag. Here you will find a jumping enemy. Direct it to the star coin by facing towards it and it will jump when the music goes "bah". To exit the area, go back up and jump out of the grey blocks to get above ground.

Welcome to Neoseeker, and enjoy your stay.

quote pumpkinmax

thanks for that
now i need help with level 4 (ghost shed )last coin .
its the only coin we cant get in the first 5 levels

I understand you need my help again. Then, it will be.
You need to go to all the red Toad Houses you can and hope that you get a shell. If you don't, go to the level and head for the area that contains the exit door. You will find a block that cycles between star, mushroom, mini mushroom, flower and shell. If you're really, really lucky, a shell will pop out. Use it and go to the exit but don't enter. Instead, keep moving to the right and you will see some grey blocks. Get some distance between yourself and the blocks and run at full speed into the blocks to smash them. You will rebound, so get some distance again and head through the gap. Collect the coin. Now exit the level, rest assured you've got all the star coins in the first five worlds.