Which statistics formula to use in Excel?

I have the following problem for my stats class and I am trying to figure out which statistical formual I need to use to get the answer. Please advice! Any help will be much appreciated.

A sample of salaries finds sample mean $35 thousand and sample variance 81. If the population mean is to be estimated within $2 thousand (that is, $2 thousand above and $2 thousand below), how many employee salaries must be sampled? Assume 95% confidence and t-multiple = 2.