Could you tell me how to prove that in $SL\_2({\mathbb{F}\_q})$ the only element of even order is $-I$ ($ \ I$ - identity matrix)?

I would really appreciate a thorough explanation, because I cannot find anything on my own.

[Here](http://www.jmilne.org/math/CourseNotes/GT310.pdf), page 29, is why I'm asking.