Using Excel, how to solve this question?

A construction company has a project to complete. There are three possible values for the number of months to complete the project, the random variable X, which are 3,4, or 5 months. Assume the probability of X is 2/6, 3/6, and 1/6 respectively. Develop the probability mass function table in excel worksheet, then compute the standard deviation of the value of X.