What formula do I use to figure this one in Statistics:

You are hired by a used car dealership to determine if the average number of miles on cars that are traded in is greater than 100 thousand miles. You collect information from the last 81 cars that have been traded, and find that the average number of miles on those cars is equal to 102 thousand miles, with a standard deviation of 9 thousand miles. Using this information, test the hypothesis that the average number of miles on these cars is greater than 100 thousand (at alpha = 0.05)