3 The heights of a certain variety of plant have been found to be normally distributed with mean 75.2 cm and standard deviation 5.7 cm . A biologist suspects that pollution in a certain region is causing the plants to be shorter than usual. He takes a random sample of \(n\) plants of this variety from this region and finds that their mean height is 73.1 cm . He then carries out an appropriate hypothesis test.

1. He finds that the value of the test statistic \(z\) is - 1.563 , correct to 3 decimal places. Calculate the value of \(n\). State an assumption necessary for your calculation.
2. Use this value of the test statistic to carry out the hypothesis test at the 6\% significance level.