Default Need help on rooting GT i9205

hello, i have a question here, i have GT i9205 Galaxy Mega and i try to root it using the modified kernel, and i success but the phone have major problem, playstore doesnt work etc..i search and find out that i need to flash back the stock kernel...where do i find that? i try many link but they all lead to stock confuse me..does i need to flash the stock kernel or the stock firmware?

sorry if it is kind of dumb question, im not pretty good with samsung phone..