Need help with the following question:

The samples come from a normal distribution, the true standard

deviation is 3. Here are some critical normal

percentiles. z_{0.9}=1.282, z_{0.95}=1.645, z_{0.975}=1.96,

z_{0.99}=2.326

(a) Compute a 95% confidence interval for population mean u

given the sample mean is 5.7, and sample size is 9.

(b) Compute a 90% confidence interval for population mean u

given the sample mean is 4.9 and sample size is 16

(c) How large the sample size n must be if the width of a

95% confidence interval is no more than 2?