How can i start 300 king quad that is stuckn in gear

I assume you rocked it back and forth to try to get it out of gear. Usually there is a lock-out that prevents the starter from operation when it is in gear. I don't know your specific model but you would have to defeat that feature to crank the engine. Not a good idea.
You really have to get it in neutral because even if you got it started you are looking at some other problem with the transmission.