You are in a company for 5 years, then a newly hired employee similar to your job functions has a high salary as compared to you. What will you do?

I will start a self-reflecting and feedback gathering process to determine why the management felt the other employee deserved a higher salary. If I find the reason something I truly lack in some area, I will work on fixing the issue and aim to improve. If there is no clear reason and the decision wasn't  based on merits, I would start looking for another company that can appreciate my efforts better.