I assume you mean you have 7 data points from one group, and 2 data points from a second group, both of which are subsets of populations (e.g. subset of males and subset of females). The maths for the t-test can be obtained from this Wikipedia page. We will assume an independent two-sample t-test, with unequal sample sizes (7 vs. 2) and unequal variances, so about half-way down that page. You can see that the calculation is based on means and standard deviations.
