z crit and z obt problem. Please Help!

**Here's the question:**

The chairperson of a psychology department believes the Ph.D. program of her department is particularly good; that it attracts exceptional graduate students, and consequently, the Ph.D. students of her department earn their Ph.D.'s in significantly less time than do their counter parts in other psychology departments. To test this belief, she compares the time it took students to earn their Ph.D.'s in her department over the past three years, with national data for the same time period. In the past three years, this department has awarded 35 Ph.D.'s; the mean time to obtain the Ph.D. was 62.0 months, with a standard deviation of 6.2 months. National data for psychology departments throughout the United States compiled for that time showed that the mean time to obtain a Ph.D. was 78.4 months, with a standard deviation of 11.7 months, and the distribution of these scores was normally distributed.

How do I find the z cit and z obt with a two tailed hypothesis test with a 0.01 significance level?

All I know is the equation for zobt is xobt-mean/standard deviation/square root of N.

There are two standard deviations, rather than one so I am confused on how to set the equation up.