Could someone please help me to complete the following query ?

Could someone please help me to complete the following query ? Also please provide the explanation/formulas for my future reference.

A doctor wants to estimate the mean HDL cholesterol of all​ 20- to​ 29-year-old females. How many subjects are needed to estimate the mean HDL cholesterol within 3 points with 99 % confidence assuming is equals 14.9 based on earlier​ studies? Suppose the doctor would be content with 90 % confidence. How does the decrease in confidence affect the sample size​ required?

A​ 99% confidence level requires  subjects. ​(Round up to the nearest​ subject.)

A​ 95% confidence level requires  subjects. ​(Round up to the nearest​ subject.)

Thanks in advance !!