How do you find the order of a special linear group? For example, how do you find order of SL(2,4)? I think the answer's 60 but really would like a proof. How do you find the order of SL(2,5)?

Finally, here's a problem set on one of my problem sheets - "Show that the only scalar matrices in SL(2,5) are I and -I. This seems fairly obvious but I'm not sure how to begin a proof. And why do you think this question was asked? (No proof required, I just want to know the reason behind asking this).