T-scores, n1, n2, and probability

Maybe this should have gone in the probability section, but I'm not sure... soo... here ya go. I hope someone can help, I have an exam tomorrow.

The question is:
For each of the following testing situations, give the p-values of the observed test statistic.
Choose the one best answer.
Two-sample t-test with conservative approach. Ho : μ1 − μ2 ≥ 0 versus Ha : μ1 − μ2 < 0,
sample sizes n1 = 16, n2 = 6 and t = 2.57.

The problem is that the prof never exactly explained how to find p. So.. can someone explain to me how to do this?