I need help with this problem !!!!!!!!   My brain is turning to mashed patato's  Thank-You

  In a study on infants, one of the characteristics measured was head circumfrence. The mean head circumference of 15 infants was 34.6 centimeters (cm).

a.) Assuming the head circumfrence for infants are normally distributed with standard deviation 2.1 cm, determine a 90 percent confidence interval for the mean head circumfrence of all infants. Question: The confidence interval for the mean head circumfrence of all infants is from ______  to ________.

b.) Obtain the margin of error, E,  for the confidence interval you found in part (a.)  Question:  What is the margin of error _______.

c.) Determine the sample size required to have a margin of error of 0.9 cm. with a 95 percent confidence level.  Question: What is the required sample size to have a margin or error of 0.9 cm with a 95 percent confidence level is ___________.