Then you must read about asymptotic notations either from an online portal or from the CLRS book.

The answer would be theta(n) as in question it is mentioned the pointer will be at starting node so in both worst and best case it will take O(n) that's why answer will become Theta(n).

Also before posting the question search it on google and on the site, someone might be already asked that question and got the answer it will save your time too. This question is already posted before here https://gateoverflow.in/170133/linked-list-time-complexity.