Confidence Interval

#1
Construct a 90% confidence interval on the population mean if a random sample of 25 items taken from a normally distributed population with a variance of 2600 had an average of 42.1

Please someone explain how to do this in great detail.
 
#2
Construct a 90% confidence interval on the population mean if a random sample of 25 items taken from a normally distributed population with a variance of 2600 had an average of 42.1
Thank you! Now I have done that.

Please someone explain how to do this in great detail.
Search on the internet. Or on this site. It is a very common question. And it has been explained in great detail on the universities and the You tube.