When I drained my kitchen sink. My bathtun had sewage come out of it's drain into tub.what do I do to fix?

More likely a clogged drain line after the tub. Have someone rod or auger the line.

I agree with ma0641. This isn't a blocked vent. It's a clogged drain line.
This can be addressed by snaking the tub drain from the overflow. Let me show you how/
Ove the overflow plate,(The one with the drain lever) and pull out the tub stopper. You now have a clear shot at the trap. The way you were going in through the drain it ran you square into a tee that you can't get around. Now feed the cable( 1/4" best or 3/3" with a small tip. You can rent a Ridgid K50 sink cable.) About 2' down you will run into the bend of the trap. Crank and push at the same time to work around it. Your blockage will be found in the lateral pipe going to the stack. You shouldn't have to put out more then 10 or 12 feet. I have found tub and shower clogs to be mostly hair and the worm should auger into it and pull it back. After you snake pour a few pans of boiling water down the drain. This will loosen the grease and flush the mess out into the main. Hope this helps and thank you for rating my reply. TOM