Lol, two options, plunger or snake, and not a sink plunger either you need a bigger one with about a 12" diameter, the snake is just like a very long flexible spring and remeber to turn it as you put it down.

You can also look and see if the pipe has any openings further down to determine where the blockage is, but, and I can't stress this enough, DON'T USE CHEMICALS as you're not a plumber and the odds are you'll just make a corosive mess above the blockage and burst the pipe.

Oh, and stop flushing the bloody thing.