Need help with this problem please.

Peak expiratory flow (PEF) is a measure of patient's ability to expel air from lungs. Patients with asthma or other respiratory conditions often have restricted PEF. The mean PEF for children free of asthma is 306. An investigator wants to test whether children with chronic bronchitis have restricted PEF. A sample of 40 children with chronic bronchitis are studied and their mean PEF is 279 with a standard deviation of 72. Alpha = 0.05

a. Critical value:

b. Computed z:

I'm confused, is critical value 1.96? how do I get computed z? Thanks.