Remember befor apllying log cancle common terms in funtions. otherwise it leads wrong answer.
Example: n3 , n2 so on take log both given O(logn).
But correct method is cancle common terms = n2 then take see n s always greter than 1 so n3 is greater than n2.
if you talk about 2n and 3n since it is seen 3n is always greater than appling log is wrong
give me case i will tell u based on that how we do .