T-Student question

[Asier]

I need to calculate one of the t-student parameters, but I don't know how to
do it with the gsl library.

If 't' is the tStudent probability, and t_X the probability with 'X' degrees
of freedom P(t_7 < 2.11) I can get it with the gsl_rand_tdist_pdf(x, nu)
function.

    double d = gsl_rand_tdist_pdf(2.11, 7);

¿Ok? How can I calculate the t_(n-1;\alpha) probability 

Given I know the above 'd' and 'nu' numbers, how can I calculate the 'x'
parameter of the function? I've seen tables with this data but I don't know
if there's a 'direct way' to calculate it with gsl.

PD: Excuse my poor english.
--
Asier