A random sample of the starting salaries of 35 randomly selected graduates with bachelor's degrees last year gave sample mean and standard deviation $41,202 and $7,621, respectively. Test whether the data provide sufficient evidence, at the 5% level of significance, to conclude that the mean starting salary of all graduates last year is less than the mean of all graduates two years before, $43,589.

Please give me a detailed explanation so I understand the concept.