hi, can someone please help me find the t critical value for this question:

A bookstore recorded the amount spent by a subset of its members for its online gift-shop. The average spent this year was $10.90, with a standard deviation of $2.10 for 510 members observed. The average spent last year was $10.30 with standard deviation $3.14 for 520 members observed. Test whether the average amount spent by members last year compared to this year is not significantly different.

So i think that we use t-test, and i can find the test statistic, but i dont know how to calculate the t critical value as im unsure of what the degrees of freedom is ( i think either 9 or 519 or 509?). Can someone please help me?