The variances appear to be different so unless you have a reason to believe the variance is equal assume that variances are unequal. Sample sizes are also unequal so you will need to use Welch's t-test.

The t statistic is calculated the same way as your normally would

The formula to calculate degrees of freedom is quite long so here's a link to it Student's t-test - Wikipedia, the free encyclopedia To be conservative you should round down the degrees of freedom. For the Welsh test the pooled standard deviation is not used, a different estimator of standard deviation is used.