I passed World 7-6, but it didn't open up World 7-7 for me. Is there a..

If you want to unlock 7-7 go back to 7-6, when you are at the halfway flag break the blocks at the top first, then after that break the middle block at the bottom that contains a vine. when you pass the moving mushroom you will reach the red flag which unlocks 7-7.

i beat 7-6 but where do i go after

you have to find the other exit in the level so you don't start a square.

You need to go to the first castle in world 7, and beat that the secret way to unlock the path to world 7-6. It is in the second stage of the castle, a hidden passage on the right on the way up through the bomb-cannons.
I just found it this morning.

I Actually unlocked world 7-7 it didn't even take me long to find the secret exit in one of the levels between 7-tower and 7- castle and you need a mini mushroom for it just bounce off the spring mushroom platform with a flying item block over it in place on the left side of the level and jump onto the flying item block and jump to the ledge of it and go up the warp pipe and you'll find thr secret exit and that's how you unlock level 7-7.