A Precision Control Engineer Is Interested in the Mean Length $\alpha$

A precision control engineer is interested in the mean length of tubing being cut automatically by machine.It is known that the standard deviation in the cutting length is 0.15 feet.A sample of 60 cut tubes yields a mean length of 12.15 feet.This sample will be used to obtain a 99% confidence interval for the mean length cut by machine.
a.What is the z $\alpha$ / 2 value to use in obtaining the confidence interval?
b.Develop the 99% confidence interval for $\mu$ .