How to calculate freelancing rate?

For more context, I’m an Android dev with 4yrs exp. I have not done freelancing before. I’m approached by a company where they need a feature implementation in existing project. I estimated efforts to be around 4-5 days and complexity is medium. How do you calculate hourly rate based on that? Experienced freelancers please also let me know any other key things I should always do or keep in mind.