Use the Poisson Distribution to find the indicated probability.

A computer salesman averages 1.5 sales per week. Use the Poisson distribution to find the probability that in a randomly selected week the number of computers sold is 2.

I'm not sure how to find the mean. Do I divide 2 with 7 days in a week or the # of sales (1.5) in a week (7)?

I really need help.