How do I solve this question? This is the entire exact question given in the assignment.

2. (a) Several million alien beings land on planet earth and they congregate on an island. They become the subject of intense scientific study. A random sample of 81 recorded deaths of the aliens showed an average lifespan of 72 years. Assuming a population standard deviation of 12 years for the alien lifespan, does the sample data seem to indicate that the mean life span of the aliens is greater than 65 years? Use a 0.01 level of significance. NOTE: Do NOT use t-distributions in your calculations (which we did not learn in the course); you can assume this is a large enough sample that normal distributions can be reliably used. (b) The aliens soon became accustomed to their new environment and start to live very happily. In a later study, a random sample of 25 recorded alien deaths over 1 year showed an average life span of 300 years. The sample standard deviation was 20 years. Find the confidence interval for the alien population's life span assuming a 95% confidence level.