I'm reading discovering knowledge in Data by Daniel T. LaRose. In the two-sample t-test for difference in means section, there is the following example:

t*data*  = x-bar(1) - (xbar2)/ sqrt(s^2(1)/n(1)+ s^2(2)/(n2) =

1.5714 - 1.5361 / 1.3126^2 + 1.3251^2 = 0.6595

The two-tailed p-value for t*data* = 0.6594 is :

p-value = 2 x p(t> 0.6595) = 0.5098

My problem is that there is no reference or explanation in the book about how the result 0.5098 came to be.