2009 DAT QR question

[myshinyteethandme]

---

Members don't see this ad.

Can anyone show me how to do #9. I just want to confirm that what I'm doing is correct.
Which of the following is equal to cosx if sec x= -5/3 and if pi/2<x<3pi/4

A. -0.28
B.-0.6
C.8/5
D.7/25
E.16/25

Thank you!!