What is the z-statistic equal to?

Please use the following information to answer the question. A researcher is interested in whether a certain supplement affects cognitive function. The researcher obtains a sample of n equal to 36 participants and gives each participant a certain dosage of the supplement. After a year of taking the supplement, the participants were give a standardized cognitive skills test. The mean score was M equal to 72.8. For the general population, the scores on the test form a normal distribution with mu equal to 70 and sigma equal to 8. Can the researcher conclude that the supplement has a significant effect on cognitive function at the significance level of 5%? What is the z-statistic equal to? O 12.6 O 3.73 O -2.1 O 2.1