In this situation you FOIL the two equations. Its complex to explain it over text so you should watch a video on how to do it (if you dont know how already)
Multiply K by 3K to get 3K^2 (^2 means squared)
Multiply K by -6 to get -6K
Multiply 4 by 3K to get 12K
Multiply 3 by -6 to get -18
Combine like terms and the final answer is:
3K^2+6K-18