咩係 Hypothesis???

請用下題佢題解教我點計﹗

2. From the latest census records, those who got their first degree from a local university four years ago have their monthly salaries normally distributed with standard deviation of $14624. If a random sample of 400 such local university graduates has a mean monthly salary of $16860, can you say that the mean monthly salaries of all those who got their first degree from a local university is not $18688?

請必須用中文教﹗